test-network: add a test case for DNSSL in RA

Closes #4590.

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

@@ -3,3 +3,6 @@ Name=veth99
 
 [Network]
 IPv6AcceptRA=true
+
+[IPv6AcceptRA]
+UseDomains=yes

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

@@ -7,6 +7,7 @@ IPv6SendRA=yes
 [IPv6SendRA]
 DNS=_link_local 2002:da8:1:0::1
 DNSLifetimeSec=1min
+Domains=hogehoge.test
 
 [IPv6Prefix]
 Prefix=2002:da8:1:0::/64

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

@@ -3314,6 +3314,10 @@ class NetworkdRATests(unittest.TestCase, Utilities):
         self.assertRegex(output, 'fe80::')
         self.assertRegex(output, '2002:da8:1::1')
 
+        output = check_output(*resolvectl_cmd, 'domain', 'veth99', env=env)
+        print(output)
+        self.assertIn('hogehoge.test', output)
+
         output = check_output(*networkctl_cmd, '-n', '0', 'status', 'veth99', env=env)
         print(output)
         self.assertRegex(output, '2002:da8:1:0')