PythonPlanBinderTest issues with python paths

There are some issues with PythonPlanBinderTest and the Python2/3 tests:

• the path is not set correctly (only inside config, not the configuration that is passed on to the PythonPlanBinder
• linux distributions have become quite inventive regarding python binary names: some offer python as Python 2, some as Python 3. Similarly, python3 and/or python2 may not be available. If we really want to test both, we need to take this into account.