[lldb] Restore original meaning to...

[lldb] Restore original meaning to test_qThreadStopInfo_only_reports_one_thread_stop_reason_during_interrupt

D119167 changed the meaning of that test by removing the use of the
interrupt packet. I did not notice this because the interrupting
happened in a shared utility function.

This patch restores the original meaning of the test, but (almost)
avoids sleeps by using process stdout to synchronize. Sadly, this means
the test stays disabled on windows, as it does not implement output
forwarding.