The test performs the following:
CREATE tbl (x INT PRIMARY KEY)
t1 t2
BEGIN BEGIN
INSERT INTO tbl VALUES (1)
SELECT * FROM tbl FOR UPDATE
That will cause t2 to wait.
INSERT INTO tbl VALUES (0)
That will cause t2 to be rolled back due to a deadlock.
Without transaction replay, the SELECT will return with an error.
With transaction replay, the deadlock error will be caught, the
transaction replayed and SELECT will return successfully.