Still chasing the mystery of the stray idle flush, let's ensure that the
heartbeat does not run at the same time as our test and confuse us.

References: https://gitlab.freedesktop.org/drm/intel/issues/541Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Andi Shyti <andi.shyti@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200227085723.1961649-8-chris@chris-wilson.co.uk