Ditch Cram

[arobrien]

Shift remaining tests from Cram to pytest.

Makes heavy use of the pytester fixture, especially pytester.run() and result.stdout.fnmatch_lines([...])

Minor changes to test_utils.py to get rid of pytest warnings.

Fixes #38
Closes #154
Closes #165

[kynan]

Thanks a lot for doing this! The tests are a little less readable, but we get a lot more flexibility at that price. And running on Windows of course, so it's worth it 👍🏽