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's very convoluted, because Windows has such crappy support for console inheritance, but if you look closely, you'll see that we don't run the ``$0`` command, we pipe it into ``runtests.sh``, and then we run that script.

The sub process in this context is the process launched by ``$GAINROOT``, which in turns calls ``runtests.sh``. The reason for doing it this way is that:
1. GAINROOT processes don't inherit the console, so to capture the output we need to pipe it to a file.
1. We cannot pass piping commands into GAINROOT either, they need to be in a separate script.