Convert test_mcsolve.py to pytest
Convert test_mcsolve.py to pytest

Parametrizes a lot of the "known" test cases using a mixin, rather than
a really long pytest_generate_tests.  I thought this would be a bit
easier to read and understand.

On several of the tests of general behaviour I reduced the number of
trajectories so the tests would run faster, and consequently removed
their slow markers.

cgranade/qutip,cgranade/qutip,qutip/qutip,qutip/qutip