tests-extra: added tests for zone serial changes

covered:
- data change, serial increase
- data change, serial doesn't change
- data change, serial < initial serial

tests-extra/tests/zone/reload/data/serial.zone

+$ORIGIN serial.
+$TTL 3600
+
+@	SOA	dns1 hostmaster 2010111213 10800 3600 1209600 7200
+	NS	dns1
+	NS	dns2
+
+dns1	A	192.0.2.1
+	AAAA	2001:DB8::1
+
+dns2	A	192.0.2.2
+	AAAA	2001:DB8::2
+
+new-record0 A 1.2.3.4 ; RR tagging the zone version (independent on serial)

tests-extra/tests/zone/reload/data/serial.zone.1

+$ORIGIN serial.
+$TTL 3600
+
+@	SOA	dns1 hostmaster 2010111214 10800 3600 1209600 7200
+	NS	dns1
+	NS	dns2
+
+dns1	A	192.0.2.1
+	AAAA	2001:DB8::1
+
+dns2	A	192.0.2.2
+	AAAA	2001:DB8::2
+
+new-record1 A 1.2.3.4 ; RR tagging the zone version (independent on serial)

tests-extra/tests/zone/reload/data/serial.zone.2

+$ORIGIN serial.
+$TTL 3600
+
+@	SOA	dns1 hostmaster 2010111214 10800 3600 1209600 7200
+	NS	dns1
+	NS	dns2
+
+dns1	A	192.0.2.1
+	AAAA	2001:DB8::1
+
+dns2	A	192.0.2.2
+	AAAA	2001:DB8::2
+
+new-record2 A 1.2.3.4 ; RR tagging the zone version (independent on serial)

tests-extra/tests/zone/reload/data/serial.zone.3

+$ORIGIN serial.
+$TTL 3600
+
+@	SOA	dns1 hostmaster 2010111212 10800 3600 1209600 7200
+	NS	dns1
+	NS	dns2
+
+dns1	A	192.0.2.1
+	AAAA	2001:DB8::1
+
+dns2	A	192.0.2.2
+	AAAA	2001:DB8::2
+
+new-record3 A 1.2.3.4 ; RR tagging the zone version (independent on serial)

tests-extra/tests/zone/reload/data/serial.zone.4

+$ORIGIN serial.
+$TTL 3600
+
+@	SOA	dns1 hostmaster 2010111212 10800 3600 1209600 7200
+	NS	dns1
+	NS	dns2
+
+dns1	A	192.0.2.1
+	AAAA	2001:DB8::1
+
+dns2	A	192.0.2.2
+	AAAA	2001:DB8::2
+
+new-record4 A 1.2.3.4 ; RR tagging the zone version (independent on serial)

tests-extra/tests/zone/reload/test.py

+#!/usr/bin/env python3
+
+'''Test for reload of a changed zone (serial up, nochange, serial down). '''
+
+from dnstest.test import Test
+from dnstest.utils import set_err, detail_log
+
+t = Test()
+
+master = t.server("knot")
+
+# Zone setup
+zone = t.zone("serial.", storage = ".")
+
+t.link(zone, master, ixfr = True)
+
+t.start()
+
+# Load zones
+serial = master.zone_wait(zone)
+
+def reload_zone(serial, version):
+    master.update_zonefile(zone, version)
+    master.reload()
+    new_serial = master.zone_wait(zone)
+    if new_serial != serial:
+        set_err("SOA MISMATCH")
+        detail_log("!Zone '%s' SOA serial %s != %s" % (zone[0].name, new_serial, serial))
+    return new_serial
+    resp = master.dig("version%d.%s" % (version, zone[0].name), "A", flags="RD")
+    resp.check(rcode="NOERROR", flags="AA")
+
+# Zone changes, serial increases (create changeset)
+version = 1 
+serial = serial + 1
+reload_zone(serial, version)
+
+# Zone changes, serial doesn't change (no new changeset)
+version += 1
+reload_zone(serial, version)
+
+# Zone changes, serial jumps out-of-range (journal is not applicable)
+version += 1
+serial = serial - 2 
+reload_zone(serial, version)
+
+# Stop master.
+master.stop()
+
+t.end()