Change test framework to pytest

[ijpulidos]

We are currently using nosetests for our testing framework and we probably want to use the capabilities of a modern framework such as pytest.

I can foresee this requiring some effort, even if it should be pretty straight forward.

[mikemhenry]

Looking at how we invoke the tests: nosetests openmmtools/tests --nocapture --verbosity=2 --with-timer --with-doctest -a '!slow'

I know we will need https://pypi.org/project/pytest-attrib/ so we can handle -a '!slow'

[mikemhenry]

We will need to convert tests that use yield into parameterized tests, which I think is the only major code change needed