drm/i915/selftests: Assert that the idle_pulse is sent

When checking the heartbeat pulse, we expect it to have been sent by the
time we have slept. We can verify this by checking the engine serial
number to see if that matches the predicted pulse serial. It will always
be true if, and only if, the pulse was sent by itself (as designed by
the test).

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/20191031094259.23028-1-chris@chris-wilson.co.uk