selftest: Don't run KCC on backup testenvs (to avoid flappiness)

KCC on the backup domain (i.e. backupfromdc, restoredc, offlinebackupdc) can establish new connections for replication. Depending on timing, this can cause the join_ldapcmp test to fail, because there's an extra object under the NTDS Settings, at the point the ldapcmp is done.

We don't need any replication to happen on the backup domain. The backup/restore workflow in the real world should mean that the restored DC is never run in the same network as the original DC.

This patch updates the default KCC command for the backup testenvs to be a no-op, so the DCs won't create new connection objects.