Set cluster_mysql_servers_sync_algorithm=3 in test_cluster_sync replica config

The replica used the default algorithm (1) which syncs runtime
mysql_servers — including monitor-shunned status. The test checks
the config table (mysql_servers) for status='ONLINE', but the
runtime sync overwrites the config table with SHUNNED status.

With auto_select (3) and monitor enabled (-M flag), the replica
uses mysql_servers_v2 sync which preserves the configured status.

test/tap/tests/test_cluster_sync_config/test_cluster_sync-t.cnf

@@ -17,6 +17,7 @@ admin_variables=
     cluster_mysql_users_diffs_before_sync=3
     cluster_admin_variables_diffs_before_sync=3
     cluster_proxysql_servers_diffs_before_sync=3
+    cluster_mysql_servers_sync_algorithm=3
     cluster_sync_interfaces=false
 }