Python on Windows inexplicably chokes on trailing slashes.

This is possible now that https://github.com/google/protobuf/issues/855
is fixed (or at least appears to be fixed).

This reorganizes the Python code, scraps the current testing
infrastructure, and implements a simple test discovery and run script
based on the standard Python unittest library so we can trust that our
tests are running.

These changes probably should have been in 3b0fefb2 but
were overlooked.

This is in preparation of moving all tests into a separate package to
enable ease of coverage checking and testing.