To bulkify the trigger, just move the SOQL query out of the for loop like this:

trigger LastSWDate on Task (after insert, after update) 
{
 
	//To do - If the subject of a completed task contains "SW", put the date of the completed task in the 
	//the "Last_SW_Activity__c" field on the account object

	//Create a set of related account ID's
	Set <ID> acctIDs = new Set <ID> ();
	//Create a list to hold the Updated Accounts
	List<Account> updAccts = new List<Account>();

	//For every task, add it's related to account ID to the set
	for (Task t: Trigger.new)
	{
		acctIDs.add(t.accountID);
	}
	//Create a map to match the task related to ID's with their corresponding account ID's
    Map<ID, Account> acctMap = new Map<ID, Account> ([Select ID, Last_SW_Activity__c from Account where ID in :acctIDs]);
	for (Task t: Trigger.new)
	{
		//Create the account object
		Account acctRec = acctMap.get(t.accountID);

		//If the account ID isn't null, the subject line starts with "sw", and the task has been marked as completed    
		if ((t.accountID != null) &&(t.subject.indexOf('sw')==0) && (t.Status == 'Completed'))
		{
			//Check to see if the Last_SW_Activity__c field is current compared with the latest completed activity
			if (acctMap.get(t.accountID).Last_SW_Activity__c < t.LastModifiedDate || acctMap.get(t.accountID).Last_SW_Activity__c ==null)
			{
				//Update the Last_SW_Activity__c field on the account object with the task's end date  
				acctrec.Last_SW_Activity__c = t.LastModifiedDate;
				updAccts.add(acctrec);
			}
		}
	}
	update updAccts;
}

 And your test class could be something like this :

@isTest
private class TestSW
{
    static testMethod void testSW() 
    {
		Integer num = 5;
        String aName = 'Test-acc-';
        List<Account> accList = new List<Account>();
        for (Integer i = 0; i < num; i++)
        {
            aName = aName + String.valueof(i);
            accList.add(new Account(Name = aName, Last_SW_Activity__c = date.today()-1));
            aName = 'Test-acc-';
        }
        insert accList;
        String tName = 'Test-task-';
        List<Task> tList = new List<Task>();
        for (Integer i = 0; i < num; i++)
        {
            tName = tName + String.valueof(i);
            tList.add(new Task(Subject = 'sw', WhoId = accList[i].Id, ActivityDate = date.today()-100, Status = 'Completed', LastModifiedDate = date.today()));
            tName = 'Test-task-';
        }
        
        Test.startTest();
        
        insert tList;
		
		Test.stopTest();
		
		// assert here
		Account a = [SELECT Last_SW_Activity__c FROM Account WHERE Name = 'Test-acc-1'];
		system.assertequals(date.today(), a.Last_SW_Activity__c);
	}
}

 I may have missed a few fields, but that is most of it I believe.

If you want to test 85000, just change num to 85000, but that may still cause an error because you may need to do a batch job instead.