test-network: add a test case for IPv6PrefixDelegation.DNS=linklocal

test/test-network/conf/ipv6-prefix.network

@@ -4,6 +4,10 @@ Name=veth-peer
 [Network]
 IPv6PrefixDelegation=yes
 
+[IPv6PrefixDelegation]
+DNS=_link_local 2002:da8:1:0::1
+DNSLifetimeSec=1min
+
 [IPv6Prefix]
 Prefix=2002:da8:1:0::/64
 PreferredLifetimeSec=1000s

test/test-network/systemd-networkd-tests.py

@@ -2646,6 +2646,11 @@ class NetworkdRATests(unittest.TestCase, Utilities):
         start_networkd()
         self.wait_online(['veth99:routable', 'veth-peer:degraded'])
 
+        output = check_output(*resolvectl_cmd, 'dns', 'veth99', env=env)
+        print(output)
+        self.assertRegex(output, 'fe80::')
+        self.assertRegex(output, '2002:da8:1::1')
+
         output = check_output(*networkctl_cmd, '-n', '0', 'status', 'veth99', env=env)
         print(output)
         self.assertRegex(output, '2002:da8:1:0')