drm/i915/selftests: Exercise some AB...BA preemption chains

Build a chain using 2 contexts (A, B) then request a preemption such
that a later A request runs before the spinner in B.

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/20190205123835.25331-1-chris@chris-wilson.co.uk