Improvements for test coverage

[rmisener]

The following improvements for test coverage have been suggested by @adi4656

Index mapping – Test coverage for index mapping is quite patchy, and this caused most of my problems over the summer. Because of how fiddly index mapping is, it would probably be a good idea for every formulation (including full space, reduced space, and partition-based) and every layer eval function to have tests with and without index mapping. (Although the eval functions aren’t important on their own, the tests for them are important because they indirectly test a lot of other Layer subclass methods that the eval functions happen to use.)

Testing the correct exceptions are raised – Tests that use unsupported ONNX nodes, layer types etc. and assert they raise the correct exceptions. (This is a large source of the missing test coverage right now.)