gnetworkmonitor: Fix use-after-free when using from another thread

When using g_network_monitor_get_default() from another thread, it’s
possible for network-changed events to be processed after an instance of
GNetworkMonitor has been disposed, causing use-after-free problems.

Fix that by moving some of the initialisation into the GInitable.init()
chain, rather than in a main context idle callback.

This includes a unit test which probabilistically reproduces the bug
(but can’t do so deterministically due to it being a race condition).

Commit amended by Philip Withnall <withnall@endlessm.com> before
pushing.

https://bugzilla.gnome.org/show_bug.cgi?id=793727

gio/tests/Makefile.am

@@ -49,6 +49,7 @@ test_programs = \
 	monitor					\
 	network-address				\
 	network-monitor				\
+	network-monitor-race			\
 	permission				\
 	pollable				\
 	proxy-test				\