tests/pm_rps: Add a new testcase to provoke the "stuck at max" bug

tests/pm_rps: Add a new testcase to provoke the "stuck at max" bug

Note that the sleep(5); to fully idle the gpu is _really_ important.
Without it the bug is not exhibited.

The issue at hand is that after gem_quiescent_gpu we are at max
(expected, since the blocking waits peg to max), but then we never go
down to a lower freq again until we're fully idle. The tiny load is
sufficient to keep the gpu at max. I've played around with this a bit
and even ridiculously low loads (like one MI_STORE per 50ms) are
enough to keep the gpu at max freq.

Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>