Test suite fixes aimed at pytest compatibility

[mdickinson]

This PR:

• Fixes the test suite so that it runs successfully under pytest. (This mainly involves changing names of objects that aren't test cases, but that pytest tries to treat as test cases out of the box.) For 3rd party packages, it's desirable that the "standard" Python tooling works.
• Updates some test modules to use observe instead of on_trait_change and to remove some fragile module-global state into the bargain.
• Consolidates some test classes that were shared across multiple test modules
• Removes some places where test modules were importing from other test modules. (I'd prefer all test modules to be independent of one another; shared test machinery can go into tests.support.)
• Uses pytest instead of unittest in the test-with-pypi GitHub Actions workflow. That means that our CI should now pick up both pytest and unittest-related issues.

Closes #537

[mdickinson]

Fixing conflicts ...