The bug is caused by a race condition between the 
INSERT DELAYED thread and the client thread's FLUSH TABLE.  The 
FLUSH TABLE does not guarantee (as is (wrongly) suggested in the 
test case) that the INSERT DELAYED is ever executed.  The 
execution of the test case will thus not be deterministic.

The fix has been to do a deterministic verification that both
threads are complete by checking the content of the table.