Rework loader and source tests

The old way of doing tests by poking and prauding the internals of buildstream is on the way out now that we have a proper approach for doing all tests with the frontend.

Testing using the bst CLI is important because it is supposed to be stable moving forward, while test cases need not assert that internal APIs remain the same, thus; moving these tests over to use the CLI to exercise features and failure modes reduces friction when refactoring the internals.