Use Unittest Asserts

[lkiesow]

This patch switches to the assert statements provided by Python's unit test framework to assure the statements are always executed and produce proper error messages in case of test failures.

[hugovk]

Alternatively, the popular pytest could be used for running tests, and it uses assert statements. (It does some smart detection of them to decide what error message to show.)

I could put together a PR if you like.