test: Fix credential-internal unit tests for developer environment

Author: lahirumaramba

test/unit/app/credential-internal.spec.ts

@@ -319,7 +319,7 @@ describe('Credential', () => {
         return;
       }
       delete process.env.GOOGLE_APPLICATION_CREDENTIALS;
-      expect((getApplicationDefault())).to.be.an.instanceof(RefreshTokenCredential);
+      expect((getApplicationDefault())).to.be.an.instanceof(ApplicationDefaultCredential);
     });
 
     it('should return a MetadataServiceCredential as a last resort', () => {
@@ -355,7 +355,7 @@ describe('Credential', () => {
       }
       delete process.env.GOOGLE_APPLICATION_CREDENTIALS;
       const c = getApplicationDefault();
-      expect(c).to.be.an.instanceof(RefreshTokenCredential);
+      expect(c).to.be.an.instanceof(ApplicationDefaultCredential);
       expect(isApplicationDefault(c)).to.be.true;
     });