Currently the CollatorFactoryMock we use in unittests (like in catalog_cache_test_fixture.cpp, can only set the collection default collation to either the simple collation or the kReverseString collation. This makes collation unittesting much more challenging since all calls to parse a collation BSONObj, will result in the kReverseString collation.

I think for more robust unittesting, it's necessary to at least support the three basic types outlined in collator_interface_mock.h.