The test case runs SHOW SLAVE HOSTS. The output of this is only stable after
all slaves have had time to register with the master; this happens
asynchroneously.

The test was waiting for the slave with server_id=3 to appear in the output,
but it was missing a similar wait for server_id=2. Thus, if server_id=2 was
much slower to connect for some reason, it could be missing from the output,
causing the test to fail.