A previous change introduced a runtests.sh script, which ran the unit

tests using rbext. Doing this as a shell script was simple, but

unfortunately it made it hard to run with different versions of Python.
This change switches this to a Python script, which can easily be run

with any version of Python. It wraps the RBExt() class, passing in the

required arguments. So long as rbext is installed with that version of

Python, it will work.

Ran the unit test suite with multiple versions of Python.