tests: fix quitting mainloop in test-networkmanager-service.py

Previously, the callbacks would return gl.mainloop.quit(), which is None
and thus the source got removed already. The later GLib.source_remove()
would thus use an invalid source-id.

Fix that, by no removing the source id from the callback.

Fixes: b1ff82eba5

tools/test-networkmanager-service.py

@@ -1392,13 +1392,12 @@ def main():
         raise AssertionError("Failure to request D-Bus name org.freedesktop.NetworkManager")
 
     # Watch stdin; if it closes, assume our parent has crashed, and exit
-    io = GLib.IOChannel(0)
-    id1 = io.add_watch(GLib.IOCondition.HUP,
-                       lambda io, condition: gl.mainloop.quit())
+    id1 = GLib.IOChannel(0).add_watch(GLib.IOCondition.HUP,
+                                      lambda io, condition: gl.mainloop.quit() or True)
 
     # also quit after inactivity to ensure we don't stick around if the above fails somehow
     id2 = GLib.timeout_add_seconds(20,
-                                   lambda: gl.mainloop.quit())
+                                   lambda: gl.mainloop.quit() or True)
 
     gl.mainloop.run()