Allow running unit tests with pytest

[ZedThree]

Some small changes to enable running the unit tests with either unittest or pytest

• use the unittest.skipIf decorator to skip tests, instead of removing them manually in run_all.py
• for the bundled test files, make sure the name is relative to the test directory
• rename the files to test_*.py

All of these mean running just pytest from the top level works, without breaking the existing testing workflow.

[ZedThree]

This could go even further, of course, and rewrite everything the tests for pytest. Pytest doesn't need classes, it just looks for functions called test_*, and has some useful built-in fixtures.