test-network: wait for bound interface to be processed by udevd

If another bound interface (dummy98) will be removed before that dummy99
is processed by udevd, then removing dummy98 in the next step makes the
target interface (test1) bring down.

Follow-up for 3e2f7c46da.

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

@@ -3176,6 +3176,7 @@ class NetworkdNetworkTests(unittest.TestCase, Utilities):
         # add another bound interface. The interface is still up.
         check_output('ip link add dummy99 type dummy')
         check_output('ip link set dummy99 up')
+        self.wait_operstate('dummy99', 'degraded', setup_state='unmanaged')
         output = check_output('ip address show test1')
         print(output)
         self.assertIn('UP,LOWER_UP', output)