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

Second retroactive test, make sure that the waiters are removed from the
global wait-tree when their seqno completes.

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/20170213171558.20942-4-chris@chris-wilson.co.uk