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.
I can't see `$0` being piped into `runtests.sh`.
I can see a command that runs `$0` being written to `runtests.sh`; I take it that's what you mean.
I'm still unclear as to how this answers my question.
Do you mean the exit I'm commenting on is run in a sub-process ?
It doesn't *appear* to be in a sub-process ...