Convert test_brmesolve.py to pytest
Convert test_brmesolve.py to pytest

This makes fairly heavy use of parametrisation to ensure tests are the
same, and removes the star-import so it's clear when we're actually
using `qutip` functionality in the tests.

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