Add Negative Tests

[R1kM]

Similarly to what is done in Charon, we should add negative tests to Aeneas, for instance to ensure to catch and test unsupported features.
One example is #166, where it would be good to ensure that the checks for mutually recursive traits are indeed satisfied, and that Aeneas fails as expected on the corresponding examples.

[Nadrieril]

Other example: #190