Update petstore tests to actually verify response's content

[macisamuele]

As result of supporting #313 I noticed that in the whole codebase we have not tests while we're checking not HTTP/200 responses, additionally I noticed that we're xfailing a big variety of petstore tests.

I would suggest to:

• improve tests ensuring that the different test cases are verified and that we're asserting against something and not just printing the response (like in https://github.com/Yelp/bravado/blob/master/tests/petstore/pet/addPet_test.py)
• remove tests

I'm personally biased against the first solution 😄