tests: Isolate directory access for gdbus-connection-flush test

When multiple tests were run in parallel, this would race on its access
to `~/.dbus-keyrings` to authenticate with the D-Bus server, since the
keyring directory was not appropriately sandboxed to the unit test.

Use `G_TEST_OPTION_ISOLATE_DIRS` to automatically isolate each unit
test’s directory usage.

This should hopefully fix the failure seen in
https://gitlab.gnome.org/GNOME/glib/-/jobs/1849524, where the following
was in the log for a test executed in parallel:
```
GDBus-DBUS_COOKIE_SHA1: Deleted stale lock file '/home/user/.dbus-keyrings/org_gtk_gdbus_general.lock'
```

Signed-off-by: Philip Withnall <pwithnall@endlessos.org>

gio/tests/gdbus-connection-flush.c

@@ -368,7 +368,7 @@ main (int   argc,
 {
   gint ret;
 
-  g_test_init (&argc, &argv, NULL);
+  g_test_init (&argc, &argv, G_TEST_OPTION_ISOLATE_DIRS, NULL);
 
   g_test_add ("/gdbus/connection/flush/busy", Fixture, NULL,
               setup, test_flush_busy, teardown);