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.