timeless <timeless@mozdev.org> [Mon, 28 Dec 2015 17:48:40 +0000] rev 27689
run-tests: skip threading for a single test
Threading is incompatible with most Python debuggers,
which makes debugging run-tests.py a real pain.
If there is only one test to run, skip using a thread for it.
Note that --debug is not compatible with debugging tests,
since it bypasses the output handling, which is where
much of the excitement is.