tests/network-monitor: Always use the dummy proxy resolver

If glib-networking is installed and built with libproxy support, this
test will use it. If a proxy is set in the environment, we might get
correctly told to go through it for certain accesses. However, this isn't
going to work, because the testsuite monkeys with the network monitor to
tell it that all addresses - including the proxy - aren't reachable.

We're trying to check if adding networks to a GNetworkMonitor works in
general. Proxies just get in the way here, so let's use the built in
dummy proxy resolver which just tells us that all URLs are directly
accessible.

https://bugzilla.gnome.org/show_bug.cgi?id=794801

gio/tests/network-monitor.c

@@ -543,6 +543,16 @@ main (int argc, char **argv)
 
   g_test_init (&argc, &argv, NULL);
 
+  /* GNetworkMonitor will resolve addresses through a proxy if one is set and a
+   * GIO module is available to handle it. In these tests we deliberately
+   * change the idea of a reachable network to exclude the proxy, which will
+   * lead to negative results. We're not trying to test the proxy-resolving
+   * functionality (that would be for e.g. glib-networking's testsuite), so
+   * let's just use the dummy proxy resolver, which always pretends the
+   * passed-in URL is directly resolvable.
+   */
+  g_setenv ("GIO_USE_PROXY_RESOLVER", "dummy", TRUE);
+
   init_test (&net127);
   init_test (&net10);
   init_test (&net192);