MULE-8124: Adding body to listener not found response, functional test c...

...ase for missing listener scenario and unit test case for missing path scenario.
That method is not intended as a helper for the status code validation. It simply encapsulates a repeated pattern throughout this test case. I'm going to add the matcher but I don't agree with the readability issue. Reading this method alone gives you an idea of what all the tests are about: sending a request and making sure the status code is the expected one.