drm/i915/selftests: Disable heartbeat around hang tests

If the heartbeat fires in the middle of the preempt-hang test, it
consumes our forced hang disrupting the test.

Reported-by: Daniel Vetter <daniel.vetter@ffwll.ch>
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/20200131130319.2998318-1-chris@chris-wilson.co.uk