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 method runs only once at the beginning, no matter how many classes are included for testing. So if these attributes (postModel and memberModel) are **not** declared as *static*, they will be assigned a value for the instance of these classes that is processed first. For all other instances (if any), these attributes remain with null.