merge config in getMockForModel

Otherwise, to override just one key, it's necessary to set all the other
defaults too
make it possible to run this test on its own

Without setting up paths to the test_app - two of the tests will fail

Verify/demonstrate how to mock a model without a model existing
bark if the model doesn't exist

Getting the following warning:

    ReflectionException: Class Mock_Foo_e187b1d1 does not have a
constructor, so you cannot pass any constructor arguments

Is a much less obvious way of saying "the class you're trying to mock
doesn't exist". Be more explicit