Now that we're using pytest more aggressively, I think we should start transitioning our tests to that style rather than continuing to use unittest. This PR removes some unnecessary uses of unittest I found.
I kept the test classes (while removing the inheritance from unittest.TestCase) where I felt like it added structure or logical grouping of tests.
I verified that pytest still finds all the tests in both this branch and master by running commands like:
```
pytest $(git diff --name-only master | grep -v windows_installer_integration_tests)
```