tests: Check for wrong fd delivery with zombie objects

Until recently, if an event attempting to deliver an fd to a zombie
object was demarshalled after the object was made into a zombie, we
leaked the fd and left it in the buffer.

If another event attempting to deliver an fd to a live object was in that
same buffer, the zombie's fd would be delivered instead.

This test recreates that situation.

While this is a ridiculously contrived way to force this race - delivering
an event from a destruction handler - I do have reports of this race
being hit in real world code.

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