drm/i915/selftests: Check for hangs mid context execution tests

Use the live_test struct to record the reset count before and compare it
at the end of the test to assert that no mystery hang occurred during the
test.

v2: Check per-engine resets as well

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20181012122404.10874-1-chris@chris-wilson.co.uk