Add a mock selftest to preempt a request and check that we cancel it,
requeue the request and then complete its execution.

v2: Error leaks no more.
Signed-off-by: NChris Wilson <chris@chris-wilson.co.uk>
Link: http://patchwork.freedesktop.org/patch/msgid/20170223074422.4125-13-chris@chris-wilson.co.ukReviewed-by: NTvrtko Ursulin <tvrtko.ursulin@intel.com>