This patch makes the tests pass running uninstalled and installed, with
python2 and python3 on linux, windows and osx.
The main gist is to use the new python-module to do the lifting done
by pythondetector, and with that add support for python2 and windows.
- Make sure to never have root folder in sys.path when running meson,
as pythondetector won't be able to access gi._overridesdir
- Generate a mesonconfig.py file that will be used by the testsuite to
know where meson generated files, making `python -m unittest` working.