Add simple tests for functions in Qobj

[MrRobot2211]

We should add some simple tests to check that everything is properly dispatched.

[AGaliciaMartinez]

Maybe there is a way of doing this with the tests in QuTiP. I was thinking that TestMul should assert False if the tested specialisations are not included in the proper dispatcher.

[jakelishman]

test_mathematics.py is meant to by unit tests that each concrete specialisation is implemented correctly. The tests of whether the dispatcher is working correctly would be somewhere else, especially because the intent is to have dispatch rules, which may mean that even an exact type match wouldn't get dispatched to the same function.