Improve API tests that make POST requests

There are API tests that issue HTTP POST requests and check the status code
returned by those HTTP requests. However, there are not tests which check
whether the just-created entities actually exist, nor whether their attributes
are correct. Add tests of this kind.
Talked to @Ichimonji10 over the phone and explained the need for asserting that the HTTP status code returned from the ``post`` is either 200 or 201 **before** trying to ``get`` the object back, as if something failed with the creation you'd get a Python error trying to look up a dictionary key.