Use pytest for testing support

[LourensVeen]

Pytest has a bunch of facilities that cover most of the functionality in amusetest.py. Since we're now using pytest to run the tests, it makes sense to remove as much of the custom stuff as we can and use standard pytest features instead. That will make it easier to work on the code for people familiar with pytest.

[stale[bot]]

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[rieder]

Completely agree