NOTE:- in your code you need to query opp record again to check below assert

opportunity OppObj= [select id , Team_Name__c from opportunity ];
System.assertEquals(OppObj.Team_Name__c,'Captain America');

Please check below post. I hope that will help you
1) http://amitsalesforce.blogspot.in/search/label/Test%20Class
2) http://amitsalesforce.blogspot.in/2015/06/salesforce-testing-best-practice.html
3) http://amitsalesforce.blogspot.in/2015/06/best-practice-for-test-classes-sample.html
Unit Test Mechanism in Salesforce

1) Single action
Test to verify that a single record produces the correct, expected result. So created the unit test data in your test class.

2) Bulk actions
Any Apex code, whether a trigger, a class or an extension, may be invoked for 1 to 200 records. You must test not only the single record case, but the bulk cases as well.

3) Restricted user
Test whether a user with restricted access to the sObjects used in your code sees the expected behavior. That is, whether they can run the code or receive error messages. Always use RunAs for to check user access on sObjects.


4) Positive behaviour
Test to verify that the expected behaviour occurs through every expected permutation, that is, that the user filled out everything correctly and did not go past the limits.

5) Negative behaviour
There are likely limits to your applications, such as not being able to add a future date, not being able to specify a negative amount, and so on. You must test for the negative case and verify that the error messages are correctly produced as well as for the positive, within the limits cases.


Please try below code i hope that will help you.

@isTest
public class TestOpportunityTeamUpdate {

    static testMethod void testStandardUseropp() 
	{
		// Create a new user with the Standard User profile
		Profile standardProf = [select id from profile where name='Standard User'];
		User su = new User(alias = 'standt', email='standarduser@testorg.com',
		emailencodingkey='UTF-8',FirstName='opp', lastname='testing', languagelocalekey='en_US',
		localesidkey='en_US', profileid = standardProf.Id,
		timezonesidkey='America/Los_Angeles', username='sample@opptest.com');
      
    // Switch current user to Standard User
		System.runAs(su) {  
			 Account acct = new Account(Name = 'Test Account');
			insert acct;
			opportunity opp=new opportunity (
				StageName = 'Prospecting',
				CloseDate = Date.today(),
				AccountId = acct.Id,
				Type='Existing Business',
				Demo_Status__c='No Show',
				Name = 'Test Opportunity Triggers'
			   );
			insert opp;
			//System.debug('value :'+opp.Team_Name__c);
			//System.assertEquals(opp.Team_Name__c,'Captain America');
		}
	}
    static testMethod void testStandardUserBulk() 
	{
		// Create a new user with the Standard User profile
		Profile standardProf = [select id from profile where name='Standard User'];
		User su = new User(alias = 'standt', email='standarduser@testorg.com',
		emailencodingkey='UTF-8',FirstName='opp', lastname='testing', languagelocalekey='en_US',
		localesidkey='en_US', profileid = standardProf.Id,
		timezonesidkey='America/Los_Angeles', username='sample@opptest.com');
      
		// Switch current user to Standard User
		System.runAs(su) 
		{  
			Account acct = new Account(Name = 'Test Account');
			insert acct;

			List<opportunity> lstOpp = new List<opportunity> ();
			For(Integer i=1 ;i<=200 ; i++)
			{
				opportunity opp=new opportunity ();
					opp.StageName = 'Prospecting';
					opp.CloseDate = Date.today();
					opp.AccountId = acct.Id;
					opp.Type='Existing Business';
					opp.Demo_Status__c='No Show';
					opp.Name = 'Test Opportunity Triggers'+i;
				lstOpp.add(opp);	   
			}
			insert lstOpp;
	
			List<opportunity> lstOpportunity = [select id from opportunity ];
			System.assertEquals(lstOpportunity.size() ,200);
		}
	}
}

Please follow below salesforce Best Practice for Test Classes :-

1. Test class must start with @isTest annotation if class class version is more than 25
2. Test environment support @testVisible , @testSetUp as well
3. Unit test is to test particular piece of code working properly or not .
4. Unit test method takes no argument ,commit no data to database ,send no email ,flagged with testMethod keyword .
5. To deploy to production at-least 75% code coverage is required
6. System.debug statement are not counted as a part of apex code limit.
7. Test method and test classes are not counted as a part of code limit
9. We should not focus on the  percentage of code coverage ,we should make sure that every use case should covered including positive, negative,bulk and single record .
Single Action -To verify that the the single record produces the correct an expected result .
Bulk action -Any apex record trigger ,class or extension must be invoked for 1-200 records .
Positive behavior : Test every expected behavior occurs through every expected permutation , i,e user filled out every correctly data and not go past the limit .
Negative Testcase :-Not to add future date , Not to specify negative amount.
Restricted User :-Test whether a user with restricted access used in your code .
10. Test class should be annotated with @isTest .
11 . @isTest annotation with test method  is equivalent to testMethod keyword .
12. Test method should static and no void return type .
13. Test class and method default access is private ,no matter to add access specifier .
14. classes with @isTest annotation can't be a interface or enum .
15. Test method code can't be invoked by non test request .
16. Stating with salesforce API 28.0 test method can not reside inside non test classes .
17. @Testvisible annotation to make visible private methods inside test classes.
18. Test method can not be used to test web-service call out . Please use call out mock .
19. You can't  send email from test method.
20.User, profile, organization, AsyncApexjob, Corntrigger, RecordType, ApexClass, ApexComponent ,ApexPage we can access without (seeAllData=true) .
21. SeeAllData=true will not work for API 23 version eailer .
22. Accessing static resource test records in test class e,g List<Account> accList=Test.loadData(Account,SobjectType,'ResourceName').
23. Create TestFactory class with @isTest annotation to exclude from organization code size limit .
24. @testSetup to create test records once in a method  and use in every test method in the test class .
25. We can run unit test by using Salesforce Standard UI,Force.com IDE ,Console ,API.
26. Maximum number of test classes run per 24 hour of period is  not grater of 500 or 10 multiplication of test classes of your organization.
27. As apex runs in system mode so the permission and record sharing are not taken into account . So we need to use system.runAs to enforce record sharing .
28. System.runAs will not enforce user permission or field level permission .
29. Every test to runAs count against the total number of DML issued in the process .


Please let us know if this post will help you
Let us know if this will help you

Thanks
Amit Chaudhary