drm/i915/selftests: Disable heartbeat around context barrier tests

As the heartbeat has the effect of flushing context barriers, this
interferes with the context barrier tests that are trying to observe
them directly. Disable the heartbeat so that the barriers are as
predictable as the test demands.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191115150841.880349-2-chris@chris-wilson.co.uk