Redmine #6171: Improve acceptance test speed on Windows.

Run the entire test suite under one single GAINROOT umbrella, instead
of launching it for each test. This dramatically improves the testing
speed on Windows.
It is called in a sub process, using GAINROOT (notice the ``$0`` being piped into ``runtests.sh``). So all the work happens there. All we have to do is to wait and exit. I'm thinking I should perhaps add an emergency escape hatch so we don't go into infinite recursion if GAINROOT doesn't do its job at elevating the privileges, since that is what we use to decide whether to go into the loop in the first place.