Use a shebang for the main test executable.

This replaces the "python test/" call with "./test/__main__py". It
may seem like a pointless change, but the former will always use
whatever "python" points to, and the latter will use the contents of
the shebang in "__main__.py". Distributions can fix the shebang on
that one file to ensure that the test suite is run with the proper
version of python.