fix unstable replication test

this test which has coverage for varoius flows of diskless master was
failing randomly from time to time.

the failure was:
[err]: diskless all replicas drop during rdb pipe in tests/integration/replication.tcl
log message of '*Diskless rdb transfer, last replica dropped, killing fork child*' not found

what seemed to have happened is that the master didn't detect that all
replicas dropped by the time the replication ended, it thought that one
replica is still connected.

now the test takes a few seconds longer but it seems stable.
