networkd-test.py: fix interface state checker

After 259125d53d, network interfaces
declared by .netdev files are created after systemd-networkd sends READY
notification. So, even when networkd is started, the netdevs may not
be created yet, and 'ip' command may fail. Let's also check the return
code of the command.

This also
- drops never worked stdout checks,
- makes the test fail if the interface is not created within the timeout.

test/networkd-test.py

@@ -960,10 +960,13 @@ exec $(systemctl cat systemd-networkd.service | sed -n '/^ExecStart=/ {{ s/^.*=/
 
         # wait until devices got created
         for _ in range(50):
-            out = subprocess.check_output(['ip', 'a', 'show', 'dev', self.if_router])
-            if b'state UP' in out and b'scope global' in out:
+            if subprocess.run(['ip', 'link', 'show', 'dev', self.if_router],
+                              stdout=subprocess.DEVNULL, stderr=subprocess.DEVNULL).returncode == 0:
                 break
             time.sleep(0.1)
+        else:
+            subprocess.call(['ip', 'link', 'show', 'dev', self.if_router])
+            self.fail('Timed out waiting for {ifr} created.'.format(ifr=self.if_router))
 
     def shutdown_iface(self):
         '''Remove test interface and stop DHCP server'''