Allow using different content for member and generic resources

When using a server that does not allow a POST of an existing resource, the associated tests fail, e.g. *testPostResponseStatusAndLocation*. Therefore, as an extension of what was requested in https://github.com/w3c/ldp-testsuite/pull/35, this new feature allows us to specify two different resources:
* Member resource
* Generic resource

The former is required in the initialization of the `MemberResourceTest` class. The second, if specified, is used in all methods that POST to the server. If any of them is not specified, the default model (Bug) is used instead.
AfterClass annotated methods will only run after the declared test method in its class. It seems that when no test is defined, the AfterClass is not triggered. So I replaced this annotation with AfterSuite, which actually always runs  in this case.