For that you need to slightly change the approach. Try out the following code

public class jobcntroller {

 

     public Job_Details__c job{ get; set; }

     public String CompanyName;

     public void setCompanyName(String CompanyName)

     {

        this.CompanyName = CompanyName;

     }

     //ApexPages.StandardController controller;   

     String s;
 

 

    public jobcontroller(ApexPages.StandardController stdController) {

        job = (Job_Details__c)stdController.getRecord();
        

    }   

 

   public PageReference mysave() {

      Schema.DescribeSObjectResult result= job_Details__c.sObjectType.getDescribe();    
 
     // controller.save();
	PageReference pageRef = null;
     try{ 
     	insert job; //insert job instance with fields populated from VF in inputFields
       	pageRef = New PageReference('/' + result.getKeyPrefix() + '/o'); 
        //eg for account it is /001/o for account list view
        pageRef.setRedirect(true);

     }catch(DmlException d){
     	//handle the exception
     }

      return pageRef;

   }

   static testMethod void testMyController() {

      //Use the PageReference Apex class to instantiate a page

        PageReference pageRef = Page.jobpage;

        //In this case, the Visualforce page named 'jobpage' is the starting point of this test method.

        Test.setCurrentPage(pageRef);

        //set the field values of job details 
         //all the field values those are entered in VF page has to be set here
        Job_Details__c jTest = new Job_Details__c(name='Test Job');
        
        //Instantiate and construct the controller class. 
		
		
        ApexPages.StandardController thecontroller = new ApexPages.StandardController(jTest);       

        jobcontroller controller = new jobcontroller(thecontroller);

        controller.setCompanyName('abc');

        //The .getURL will return the page url the Save() method returns.

        String nextPage = controller.mysave().getUrl();

        //Check that the save() method returns the proper URL.

        System.assertEquals('/apex/failure?error=noParam', nextPage);

 

     }

}

 Please note the code changes highlighted in red.

Hope this approach works for you.