It doesn't save much time (maybe 0.5s out of 12+s).
I'm leaving the option in because I think it is still useful for
iteratively testing a single test case.
(imported from commit a0ac43f4c48eec101f05d731740394b30a15773b)
This now allows e.g.
./tools/test-backend zephyr.BugdownTest
or
./tools/test-backend zephyr.BugdownTest --verbosity=2
but unfortunately not
./tools/test-backend --verbosity=2 zephyr.BugdownTest
(imported from commit 95302db3efe45182d789c40c2a2899230a06b091)