I have a site that is being used internally for our team to submit cases to our admin team. I am trying to write a test class for the apex part of it but my code coverage seems to still be at 0%. Was wondering if someone might be able to help me with this as this is my first test class. 

VF: 

<apex:page standardController="Case" extensions="case_attachment">
<apex:form id="frm">
    <apex:pageBlock title="New Salesforce Case">
        <apex:pageBlockSection columns="1"  title="Salesforce Case -- Red lines represent required fields." collapsible="false">
        </apex:pageBlockSection>
        <strong>Please click the lookup icon (small button to right of field) to search name selection from Salesforce lookup dialog.</strong>
        <apex:pageBlockSection columns="1">
           <apex:inputField value="{!Case.Requestor_Name__c}" required="true"/>
           <br></br>
           </apex:pageBlockSection>
        <strong>Select Problem and High Priority ONLY if the issue is preventing you from continuing work in Salesforce.</strong>
        <apex:pageBlockSection columns="1">
           <apex:inputField value="{!Case.Type}" required="true"/>
           <apex:inputField value="{!Case.Priority}" required="true"/>
        </apex:pageBlockSection>
        <apex:pageBlockSection columns="1" title="Case Details" collapsible="false">
           <apex:inputField value="{!Case.Department__c}" required="true"/>
           <apex:inputField value="{!Case.Salesforce_Object__c}" required="true"/>
           <apex:inputField value="{!Case.Description}" required="true" style="width:50%; height: 60px" />
        </apex:pageBlockSection>
        <apex:pageBlockButtons >
           <apex:commandButton action="{!save}" value="Submit"/>
           <apex:commandButton action="{!cancel}" value="Cancel"/>
       </apex:pageBlockButtons>
    <apex:pageBlockSection title="Upload the Attachment" collapsible="false" dir="LTR" columns="1">
    <div id="upload" class="upload">                                   
        <apex:inputFile id="fileToUpload" value="{!fileBody}" filename="{!fileName}" styleClass="input-file"/>                            
    </div>
    </apex:pageBlockSection>
   </apex:pageBlock>
</apex:form>
</apex:page>

The controller that I am using for the attachment is being used to save the record, add the attachment and then the page is redirected to a thank you page. 

Apex: 

public class case_attachment
{
public case objcase{get;set;}
public Attachment myAttachment{get;set;}
public string fileName{get;set;}
public Blob fileBody{get;set;}
 
    public case_attachment(Apexpages.standardcontroller controller)
    {
        objcase = (Case)controller.getRecord();
        myAttachment = new Attachment();
    }
    public pagereference save()
    {
      if(myAttachment.Name == null)
        {
        insert objcase;
        }
      if(myAttachment.Name != null)
      {
        insert objcase;
        System.debug('@@@@@fileBody'+fileBody);    
        myAttachment = new Attachment();
              Integer i=0;
              myAttachment .clear();
              myAttachment.Body = fileBody;
              myAttachment.Name = fileName ;
              myAttachment.ParentId = objcase.id;            
              insert myAttachment;   
        }             
        pagereference pr = Page.Thank_You;                          
        pr.setRedirect(true);
        return pr;
    }
}

I was under the impression that I had to make sure that the test class ran through the required fields of the external site but it doesn't seem to show any test coverage. 

Test Class: 

@isTest
private class Test_case_attachment {
	
	@isTest static void createCase() {
		Case c = new Case();
		c.Requestor_Name__c = '0035C000001njRp';
		c.Type = 'Problem';
		c.Priority = '1';
		c.Department__c = 'Marketing';
		c.Salesforce_Object__c = 'Accounts';
		c.Description = 'this is a test submission';

		insert c;
	}
	
}

Does anyone know what I am doing wrong?