Added tests for TTL mismatch on zone load.

Both master and slave scenarios. Master should fail, slave should
load the zone.

Later, log messages should be checked too, but that's not possible now.

tests-extra/tests/zone/ttl-master/data/ttl-mismatch.zone

+$ORIGIN ttl-mismatch.
+$TTL 3600
+
+@	SOA	dns1 hostmaster 2010111201 10800 3600 1209600 7200
+	NS	dns1
+	NS	dns2
+	MX	10 mail
+
+ttl	A	192.0.2.1
+	A	192.0.3.1
+	7200 A  192.0.4.1
+	AAAA	2001:DB8::1
+	AAAA	2001:DB9::1
+

tests-extra/tests/zone/ttl-master/test.py

+#!/usr/bin/env python3
+
+'''Test for loading of zone containing mismatched TTLs'''
+
+from dnstest.test import Test
+
+t = Test()
+
+master = t.server("knot")
+
+zone = t.zone("ttl-mismatch", storage=".")
+
+t.link(zone, master)
+
+t.start()
+
+# Just check if the zone was loaded. It should be refused on master.
+resp = master.dig(zone, "SOA")
+resp.check(rcode="REFUSED", flags="QR", noflags="AA TC AD RA")
+
+t.end()

tests-extra/tests/zone/ttl-slave/data/ttl-mismatch.zone

+; This zone file is actually not used in the test,
+; it's just a placeholder to distinguish master zone
+; and updated slave zone.
+$ORIGIN ttl-mismatch.
+$TTL 3600
+
+@	SOA	dns1 hostmaster 2010111201 10800 3600 1209600 7200
+	NS	dns1
+	NS	dns2
+	MX	10 mail
+
+ttl	A	192.0.2.1
+	A	192.0.3.1
+	AAAA	2001:DB8::1
+	AAAA	2001:DB9::1
+

tests-extra/tests/zone/ttl-slave/data/ttl-mismatch.zone.1

+$ORIGIN ttl-mismatch.
+$TTL 3600
+
+@	SOA	dns1 hostmaster 2010111201 10800 3600 1209600 7200
+	NS	dns1
+	NS	dns2
+	MX	10 mail
+
+ttl	A	192.0.2.1
+	A	192.0.3.1
+	7200 A  192.0.4.1
+	AAAA	2001:DB8::1
+	AAAA	2001:DB9::1
+

tests-extra/tests/zone/ttl-slave/test.py

+#!/usr/bin/env python3
+
+'''Test for mismatched TTLs handling on slave zone load.'''
+
+from dnstest.test import Test
+
+t = Test()
+
+master = t.server("dummy")
+slave = t.server("knot")
+
+zone = t.zone("ttl-mismatch.", storage=".")
+
+t.link(zone, master, slave)
+
+# Create invalid zone file.
+slave.update_zonefile(zone, version=1)
+
+t.start()
+
+t.sleep(1)
+
+t.end()