tests/common/timeout_order: reset test case thread to original prio

That was an oversight that should not have caused any problem. However,
the test harness expects the test's thread to be higher prio than the
test suite's thread that spawns it, because the test suite reuses the
stack space for the next test, if there is one.

Change-Id: Iad951118278abf0d9c23012d78ed56b75bc2958a
Signed-off-by: Benjamin Walsh <walsh.benj@gmail.com>