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.
This class is responsible for creating its required resource, not `LdpTest`. This way, the latter will only have to worry about creating (if required by parameter) the generic resource for POSTs.