test-network: add test case for issue #30403

(cherry picked from commit 5e46ca9)
(cherry picked from commit fbe064c)
(cherry picked from commit d948b16)
(cherry picked from commit 29c975f)

Author: yuwata

test/test-network/conf/25-route-static.network

@@ -98,3 +98,10 @@ MultiPathRoute=2001:1234:5:9fff:ff:ff:ff:ff@dummy98 5
 Destination=192.168.10.2/32
 MultiPathRoute=2001:1234:5:8fff:ff:ff:ff:ff@dummy98 10
 MultiPathRoute=2001:1234:5:9fff:ff:ff:ff:ff@dummy98 5
+
+[Address]
+Address=1.1.8.105/31
+Peer=1.1.8.104/31
+
+[Route]
+Gateway=1.1.8.104

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

@@ -2684,6 +2684,7 @@ def _test_route_static(self, manage_foreign_routes):
         self.assertIn('default via 149.10.125.65 proto static onlink', output)
         self.assertIn('default via 149.10.124.64 proto static', output)
         self.assertIn('default proto static', output)
+        self.assertIn('default via 1.1.8.104 proto static', output)
 
         print('### ip -4 route show table local dev dummy98')
         output = check_output('ip -4 route show table local dev dummy98')