test-network: add test for small MTU for vcan

Prompted by https://github.com/systemd/systemd/issues/30140#issuecomment-1837973580.

(cherry picked from commit 470a329d9849d108e28f72d00dd130d130cebb01)
(cherry picked from commit 16f193d51861db4befd6228b773cf7a86c03339a)
(cherry picked from commit 6f584de56e86170c8e4bd426a4b4829b8343f1c0)
(cherry picked from commit 1cdd0c2804c9e991d5d083a32da36cc69a0b78dc)

test/test-network/conf/25-vcan.netdev

@@ -2,3 +2,4 @@
 [NetDev]
 Name=vcan99
 Kind=vcan
+MTUBytes=16

test/test-network/conf/25-vcan98.netdev

@@ -0,0 +1,4 @@
+# SPDX-License-Identifier: LGPL-2.1-or-later
+[NetDev]
+Name=vcan98
+Kind=vcan

test/test-network/conf/25-vcan98.network

@@ -0,0 +1,6 @@
+# SPDX-License-Identifier: LGPL-2.1-or-later
+[Match]
+Name=vcan98
+
+[Link]
+MTUBytes=16

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

@@ -1520,10 +1520,20 @@ class NetworkdNetDevTests(unittest.TestCase, Utilities):
 
     @expectedFailureIfModuleIsNotAvailable('vcan')
     def test_vcan(self):
-        copy_network_unit('25-vcan.netdev', '26-netdev-link-local-addressing-yes.network')
+        copy_network_unit('25-vcan.netdev', '26-netdev-link-local-addressing-yes.network',
+                          '25-vcan98.netdev', '25-vcan98.network')
         start_networkd()
 
-        self.wait_online(['vcan99:carrier'])
+        self.wait_online(['vcan99:carrier', 'vcan98:carrier'])
+
+        # https://github.com/systemd/systemd/issues/30140
+        output = check_output('ip -d link show vcan99')
+        print(output)
+        self.assertIn('mtu 16 ', output)
+
+        output = check_output('ip -d link show vcan98')
+        print(output)
+        self.assertIn('mtu 16 ', output)
 
     @expectedFailureIfModuleIsNotAvailable('vxcan')
     def test_vxcan(self):