Updating Associated Metadata Provider to follow the existing pattern.

cc @dougbu @rynowak 
we use `Mock<T>` when the test class or method has lots of complex inputs that need to be abstracted out of the unit tests.  `ModelMetadataTest` has a few older tests that are conditional but they should really be using `EmptyModelMetadataProvider` instead of a `Mock`.  that wasn't an intended precedent.