MTN/TST: move to pytest

[CJ-Wright]

Should we move tests to pytest? Many other of our dependencies have made this move and I think it will make writing tests easier.

[chiahaoliu]

currently I use py.test inside tests/, but I guess you meant a more sophisticated, python run_test.py?

[CJ-Wright]

I meant more using their test fixtures rather than unit tests objects. For example building a beamtime test fixture rather than building it in a setup method.

[sbillinge]

Let's do it.

[st3107]

Done.