test: fix networkd-test.py rate limiting and dynamic user

- Reset systemd-networkd.service before each test run, to avoid running
   into restart limits.
 - Our networkd-test-router.service unit needs to run as root and thus
   can't use `User=`; but networkd still insists on the
   `systemd-network` system user to exist, so create it.

test/networkd-test.py

@@ -54,6 +54,11 @@ def setUpModule():
     # Ensure the unit directory exists so tests can dump files into it.
     os.makedirs(NETWORK_UNITDIR, exist_ok=True)
 
+    # create static systemd-network user for networkd-test-router.service (it
+    # needs to do some stuff as root and can't start as user; but networkd
+    # still insists on the user)
+    subprocess.check_call(['adduser', '--system', '--no-create-home', 'systemd-network'])
+
 
 class NetworkdTestingUtilities:
     """Provide a set of utility functions to facilitate networkd tests.
@@ -172,6 +177,7 @@ Name=mybridge
 DNS=192.168.250.1
 Address=192.168.250.33/24
 Gateway=192.168.250.1''')
+        subprocess.check_call(['systemctl', 'reset-failed', 'systemd-networkd'])
         subprocess.check_call(['systemctl', 'start', 'systemd-networkd'])
 
     def tearDown(self):
@@ -257,6 +263,8 @@ class ClientTestBase(NetworkdTestingUtilities):
         self.assertTrue(out.startswith('-- cursor:'))
         self.journal_cursor = out.split()[-1]
 
+        subprocess.check_call(['systemctl', 'reset-failed', 'systemd-networkd'])
+
     def tearDown(self):
         self.shutdown_iface()
         subprocess.call(['systemctl', 'stop', 'systemd-networkd'])