tests/fprintd: Do not hang if we error on name appeared callback

But instead only wait for name to appear and do the tests in the main
function so that we can properly check the exception and depending on
its type skip the test or raise it so that it can be caught by the test
suite

tests/fprintd.py

@@ -493,28 +493,40 @@ class FPrintdManagerPreStartTests(FPrintdTest):
             self.manager.GetDefaultDevice()
 
     def test_manager_get_devices_on_name_appeared(self):
-        self._appeared_res = []
+        self._appeared_name = None
 
         def on_name_appeared(connection, name, name_owner):
-            self._appeared_res.append(connection.call_sync('net.reactivated.Fprint',
-                '/net/reactivated/Fprint/Manager',
-                'net.reactivated.Fprint.Manager',
-                'GetDefaultDevice', None, None,
-                Gio.DBusCallFlags.NO_AUTO_START, 500, None))
+            self._appeared_name = name
+
+        def on_name_vanished(connection, name):
+            self._appeared_name = 'NAME_VANISHED'
 
         id = Gio.bus_watch_name_on_connection(self.dbus,
             'net.reactivated.Fprint', Gio.BusNameWatcherFlags.NONE,
-            on_name_appeared, None)
+            on_name_appeared, on_name_vanished)
+        self.addCleanup(Gio.bus_unwatch_name, id)
 
         self.daemon_start()
-        while not self._appeared_res:
+        while not self._appeared_name:
             ctx.iteration(True)
 
-        self.assertIsNotNone(self._appeared_res[0])
-        dev_path = self._appeared_res[0][0]
-        self.assertTrue(dev_path.startswith('/net/reactivated/Fprint/Device/'))
+        self.assertEqual(self._appeared_name, 'net.reactivated.Fprint')
 
-        Gio.bus_unwatch_name(id)
+        try:
+            appeared_device = self.dbus.call_sync(
+                'net.reactivated.Fprint',
+                '/net/reactivated/Fprint/Manager',
+                'net.reactivated.Fprint.Manager',
+                'GetDefaultDevice', None, None,
+                Gio.DBusCallFlags.NO_AUTO_START, 500, None)
+        except GLib.GError as e:
+            if 'net.reactivated.Fprint.Error.NoSuchDevice' in e.message:
+                self.skipTest("Need virtual_image device to run the test")
+            raise(e)
+
+        self.assertIsNotNone(appeared_device)
+        [dev_path] = appeared_device
+        self.assertTrue(dev_path.startswith('/net/reactivated/Fprint/Device/'))
 
 
 class FPrintdVirtualDeviceTest(FPrintdVirtualDeviceBaseTest):