test-network: add one more test for netdev dropin .conf files

For netdev, config files are loaded twice, and the first time,
only Match and NetDev setions are read. So, the test given by
the previous commit covers only the second loading.
This adds another test that also covers the first loading.

test/test-network/conf/10-dropin-test.netdev

@@ -0,0 +1,4 @@
+[NetDev]
+Name=hoge
+Kind=dummy
+MACAddress=00:50:56:c0:00:18

test/test-network/conf/10-dropin-test.netdev.d/mac.conf

@@ -0,0 +1,2 @@
+[NetDev]
+MACAddress=00:50:56:c0:00:28

test/test-network/conf/10-dropin-test.netdev.d/name.conf

@@ -0,0 +1,2 @@
+[NetDev]
+Name=dropin-test

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

@@ -153,7 +153,7 @@ class NetworkdNetDevTests(unittest.TestCase, Utilities):
     links =['bridge99', 'bond99', 'bond99', 'vlan99', 'test1', 'macvtap99',
             'macvlan99', 'ipvlan99', 'vxlan99', 'veth99', 'vrf99', 'tun99',
             'tap99', 'vcan99', 'geneve99', 'dummy98', 'ipiptun99', 'sittun99', '6rdtun99',
-            'gretap99', 'vtitun99', 'vti6tun99','ip6tnl99', 'gretun99', 'ip6gretap99', 'wg99']
+            'gretap99', 'vtitun99', 'vti6tun99','ip6tnl99', 'gretun99', 'ip6gretap99', 'wg99', 'dropin-test']
 
     units = ['25-bridge.netdev', '25-bond.netdev', '21-vlan.netdev', '11-dummy.netdev', '21-vlan.network',
              '21-macvtap.netdev', 'macvtap.network', '21-macvlan.netdev', 'macvlan.network', 'vxlan.network',
@@ -163,7 +163,7 @@ class NetworkdNetDevTests(unittest.TestCase, Utilities):
              '25-gre-tunnel.netdev', '25-gretap-tunnel.netdev', '25-vti-tunnel.netdev', '25-vti6-tunnel.netdev',
              '12-dummy.netdev', 'gre.network', 'ipip.network', 'ip6gretap.network', 'gretun.network',
              'ip6tnl.network', '25-tap.netdev', 'vti6.network', 'vti.network', 'gretap.network', 'sit.network',
-             '25-ipip-tunnel-independent.netdev', '25-wireguard.netdev', '6rd.network']
+             '25-ipip-tunnel-independent.netdev', '25-wireguard.netdev', '6rd.network', '10-dropin-test.netdev']
 
     def setUp(self):
         self.link_remove(self.links)
@@ -172,6 +172,17 @@ class NetworkdNetDevTests(unittest.TestCase, Utilities):
         self.link_remove(self.links)
         self.remove_unit_from_networkd_path(self.units)
 
+    def test_dropin(self):
+        self.copy_unit_to_networkd_unit_path('10-dropin-test.netdev')
+
+        self.start_networkd()
+
+        self.assertTrue(self.link_exits('dropin-test'))
+
+        output = subprocess.check_output(['ip', 'link', 'show', 'dropin-test']).rstrip().decode('utf-8')
+        print(output)
+        self.assertRegex(output, '00:50:56:c0:00:28')
+
     def test_bridge(self):
         self.copy_unit_to_networkd_unit_path('25-bridge.netdev')
         self.start_networkd()