The test was changing wsrep_on option in node_3, which is native
MariaDB server (i.e. not a cluster node). Native NariaDB server
should not manipulate wsrep replication state, this problem is fixed.

galera.galera_slave_replay test phase 2 will cause certification failure
for async slave SQL handler thread. This certification failure is now
monitored and required to happen in the test.

The test phase 2, generates scenario, where async slave SQL handler faces
certification failure and galera slave applier is paused when this happens.
This makes the test vulnerable for anomaly described in MDEV-22632.
Therefore the fix in this commit depends on MDEV-22632, and should be merged
after the fix for MDEV-22632.