drm/i915/selftests: Exercise dynamic reclocking with RPS

After having testing all the RPS controls individually, we need to take
a step back and check how our RPS worker integrates them to perform
dynamic GPU reclocking. So do that by submitting a spinner and wait and
see what happens.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200420172739.11620-6-chris@chris-wilson.co.uk