libnm/tests: Properly wait for devices in /libnm/activate-virtual

Ignore the signal if we're signalled before the second device is available and
wait for another one.

libnm/tests/test-nm-client.c

@@ -888,6 +888,8 @@ client_devices_changed_cb (GObject *client,
 
 	devices = nm_client_get_devices (NM_CLIENT (client));
 	g_assert (devices != NULL);
+	if (devices->len < 2)
+		return;
 	g_assert_cmpint (devices->len, ==, 2);
 
 	if (NM_IS_DEVICE_VLAN (devices->pdata[0]))