Fix the gdbus-proxy async test

The async test had several problems:
- It created a proxy and did not launch a main loop, meaning that its
  callback would usually not get called, or, if it did get called, the
test harness would have taken down the connection already, causing an
assertion failure when the proxy had an error.
- It was dependent on the proxy test to set up the server and would fail
  because some properties were modified by that test.

https://bugzilla.gnome.org/show_bug.cgi?id=674805