tests: Add a test for fd leaks on zombie objects

Until recently, if a client destroying a resource raced with the
server generating an event on that resource that delivered a file
descriptor, we would leak the fd.

This tests for a leaked fd from that race condition.

Reviewed-by: Daniel Stone <daniels@collabora.com>
Signed-off-by: Derek Foreman <derekf@osg.samsung.com>