drm/i915/selftests: Add tests for timeslicing virtual engines

Make sure that we can execute a virtual request on an already busy
engine, and conversely that we can execute a normal request if the
engines are already fully occupied by virtual requests.

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/20200519132046.22443-1-chris@chris-wilson.co.uk