Fix some assert calls

I'm not really sure how this happened, but I'm glad I caught it and that everything was okay.
Use more semantic asserts

Rather than using `assertTrue()` and `assertFalse()` for most of the checks, the more semantic methods in `unittest` are being introduced. These include `assertIn()`, `assertIsInstance()`, `assertIsNone()` and their corresponding inverse checks. Because all test modules already import `unittest2` when possible, there is no problem using these methods which are normally only available in Python 2.7+.