test-network: disable restart limiting for networkd

Some of the tests restart networkd rapidly, so restart limiting
must be disabled to avoid start ratelimiting from causing test
failures.

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

@@ -142,6 +142,8 @@ def setUpModule():
             running_units.append(u)
 
     drop_in = [
+        '[Unit]',
+        'StartLimitIntervalSec=0',
         '[Service]',
         'Restart=no',
         'ExecStart=',