MTR drops the datadir in the event of a test failure. In case mysqld
is running as a Galera node and a failure occurs while the SST is in
progress, the rsync pid file gets removed as part of the cleanup and
wsrep_sst_rsync, which relies on this file, fails to kill the rsync.
Fixed by using the cached $RSYNC_REAL_PID to kill rsync daemon.