drm/i915/selftests: Setup engine->retire for mock_engine

Enable and cleanup the engine->retire for the mock engine.

Fixes: dc93c9b6

 ("drm/i915/gt: Schedule request retirement when signaler idles")
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Acked-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191221001136.720154-1-chris@chris-wilson.co.uk