tests-extra: backport axfr/failover test from master

tests-extra/tests/axfr/failover/test.py

@@ -24,7 +24,7 @@ master1 = t.server("knot")
 master2 = t.server("bind")
 slave = t.server("knot")
 
-# flush zones immediatelly
+# flush zones immediately
 for server in [master1, master2, slave]:
     slave.zonefile_sync = "0"
 
@@ -53,6 +53,7 @@ for server in [master1, master2]:
 
 # failover to second master
 master1.stop()
+slave.clean(zone=False, timers=True)
 slave.start()
 slave.zone_wait(zone, serial=20, equal=True, greater=False)
 master1.start()