test-network: add test cases for IPv6ProxyNDP=no

In that case, IPv6 proxy neighbor entries are ignored.

This also checks IPv6ProxyNDP=yes is implied if an IPv6 proxy neighbor
address is specified.

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

@@ -5370,9 +5370,30 @@ class NetworkdNetworkTests(unittest.TestCase, Utilities):
 
         output = check_output('ip neighbor show proxy dev dummy98')
         print(output)
-        for i in range(1, 5):
+        for i in range(1, 6):
             self.assertRegex(output, f'2607:5300:203:5215:{i}::1 *proxy')
 
+        # When a proxy NDP address is specified, IPv6ProxyNDP=yes is implied.
+        self.check_ipv6_sysctl_attr('dummy98', 'proxy_ndp', '1')
+
+        with open(
+            os.path.join(network_unit_dir, '25-ipv6-proxy-ndp.network'), mode='a', encoding='utf-8'
+        ) as f:
+            f.write('[Network]\nIPv6ProxyNDP=no\n')
+
+        stop_networkd()
+        remove_link('dummy98')
+        start_networkd()
+        self.wait_online('dummy98:routable')
+
+        # If IPv6ProxyNDP= is disabled, proxy NDP addresses are ignored.
+        output = check_output('ip neighbor show proxy dev dummy98')
+        print(output)
+        for i in range(1, 6):
+            self.assertNotRegex(output, f'2607:5300:203:5215:{i}::1 *proxy')
+
+        self.check_ipv6_sysctl_attr('dummy98', 'proxy_ndp', '0')
+
     def test_ipv4_proxy_arp(self):
         copy_network_unit('25-ipv4-proxy-arp.network', '12-dummy.netdev')
         start_networkd()