tests-extra: improve ttl-slave test

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/test.py

@@ -2,6 +2,9 @@
 
 '''Test for mismatched TTLs handling on slave zone load.'''
 
+'''NOTE: dnspython can't keep different TTLs in one rrset. So we can't check
+         the slave server properly.'''
+
 from dnstest.test import Test
 
 t = Test()
@@ -9,7 +12,7 @@ t = Test()
 master = t.server("dummy")
 slave = t.server("knot")
 
-zone = t.zone("ttl-mismatch.", storage=".")
+zone = t.zone("ttl-mismatch.", storage=".", exists=False)
 
 t.link(zone, master, slave)
 
@@ -18,6 +21,8 @@ slave.update_zonefile(zone, version=1)
 
 t.start()
 
-t.sleep(1)
+# Check if the zone was loaded.
+resp = slave.dig("ttl.ttl-mismatch.", "A")
+resp.check(rcode="NOERROR", flags="QR AA", noflags="TC AD RA")
 
 t.end()