drm/i915/selftests: Flush the submission, not cancel it!

Use intel_engine_flush_submission() when we want to ensure that the
tasklet is run. tasklet_kill(), while it may ensure that an ongoing
tasklet is completed, also prevents the tasklet from running if it's
already scheduled and hasn't yet been run.

Closes: https://gitlab.freedesktop.org/drm/intel/-/issues/1874


Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200521124304.3157692-1-chris@chris-wilson.co.uk