Add base test class for testing all backends

This base test class stub should allow migration of all backend tests
to the same test code suite. In cases where individual databases have
differing functionality, the backends will need to be more robust about
communicating capabilities to the test. Furthermore, with by adding
another forAll() wrapper, testing all combinations of database sources
and targets supported by DataExpress should be possible. If this
procedure works well, it might be possible to extend it to all other
test cases where it makes sense.