drm/i915: Add unit tests for the breadcrumb rbtree, wakeups

Third retroactive test, make sure that the seqno waiters are woken.

v2: Smattering of comments, rearrange code
v3: Fix IDLE assert to avoid startup/sleep races

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170213171558.20942-5-chris@chris-wilson.co.uk