tests: thread_metric: Fix cooperative for SMP

The thread metric cooperative benchmark had a subtle flaw on
SMP enabled systems. The reporting/main thread was running at
a higher priority than the "cooperative" threads. When that
reporting thread woke up from its 30 second sleep, there was a
chance that it would change the ordering of the "cooperative"
threads before the test expected it.

This ordering change is not present on UP systems as the current
thread is always in the ready queue. However, on SMP systems the
current thread is not in the ready queue and is re-added to the
end of the list when it is preempted by a higher priority thread.

To work around this, we make the priority of the main/reporting
thread to be the same as the "cooperative" threads. Thus when
the reporting thread wakes, it is added to the end of list and
no longer introduces an unexpected schedule point.

Signed-off-by: Peter Mitsis <peter.mitsis@intel.com>