drm/i915/selftests: Prevent the timeslice expiring during suppression tests

When testing whether we prevent suppressing preemption, it helps to
avoid a time slice expiring prematurely.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=111108
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/20190812091045.29587-2-chris@chris-wilson.co.uk