test: Add comprehensive tests for MultiPathRoute device-only syntax

Add unit tests for config_parse_multipath_route() covering the new
device-only nexthop syntax (@device). Tests verify basic parsing of
device-only routes, gateway with device routes, gateway-only routes,
interface index handling, etc. Also some checks on new hash/duplicate
semantics.

Also add an integration test to verify device-only multipath routes are
correctly installed in the kernel routing table.

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

@@ -115,3 +115,8 @@ Peer=1.1.8.104/31
 
 [Route]
 Gateway=1.1.8.104
+
+[Route]
+Destination=192.168.20.0/24
+MultiPathRoute=@test1 15
+MultiPathRoute=@dummy98 25

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

@@ -4391,6 +4391,13 @@ class NetworkdNetworkTests(unittest.TestCase, Utilities):
         self.assertIn('via 2001:1234:5:8fff:ff:ff:ff:ff dev dummy98', output)
         self.assertIn('via 2001:1234:5:9fff:ff:ff:ff:ff dev dummy98', output)
 
+        print('### ip route show 192.168.20.0/24')
+        output = check_output('ip route show 192.168.20.0/24')
+        print(output)
+        self.assertIn('192.168.20.0/24 proto static', output)
+        self.assertIn('nexthop dev test1 weight 15', output)
+        self.assertIn('nexthop dev dummy98 weight 25', output)
+
         check_json(networkctl_json())
 
     def _check_unreachable_routes_removed(self):