After setting up the copy operations, add a hanging batch. This should
mean that we complete the copy and the compare then races against the
GEM reset. Hopefully, this will catch driver bugs where the target
object is no longer accessible after the hang.

Note: hang injection is disabled until the required kernel interface is
completed. But there are useful additional tests here...

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>