drm/i915/selftests: Drain the freedlists between exec passes

During the context execution tests, we issue a lot of work and discard a
lot of objects without releasing the lock and allowing the background
reaper to free those objects. Insert a small break between each pass to
flush the worker.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190704165317.21060-1-chris@chris-wilson.co.uk