test(apps): improve app tests
test(apps): improve app tests

Improve the app system tests by adjusting and improving the test cases.

* Add additional checks to verify that the Marathon response is as expected. To do so the test will open the service detail, hit edit and check the form content.
* Introduce new test to check that Marathon throws an error in case an app with an existig ID already exists.
* Adjust create simple app test to bail early in case of an error.

dcos/dcos-ui,dcos/dcos-ui,dcos/dcos-ui,dcos/dcos-ui,dcos/dcos-ui