Revise testing emulators to the subclass and multioutput

[sgreenbury]

Might be worth drawing up an updated inheritance tree diagram for our Emulators, and also thinking about what level(s) we should be applying testing. I guess here you are doing the basic output type assertions in the classes themselves but would we want separate tests for each of these? - at the moment in test_experimental_base.py we just have TestPyTorchBackend

Originally posted by @edwardchalstrey1 in #561 (comment)

To reduce repetition across tests to each emulator, revise tests to be per subclass (add following #542)