tests: Isolate directory access for gdbus-non-socket 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/pwithnall/glib/-/jobs/1879558.
```
228/266 glib:gio / gdbus-non-socket             FAIL     8.64 s (killed by signal 6 SIGABRT)
…
--- stderr ---
GDBus-DBUS_COOKIE_SHA1: Deleted stale lock file '/home/user/.dbus-keyrings/org_gtk_gdbus_general.lock'
**
GLib-GIO:ERROR:../gio/tests/gdbus-non-socket.c:253:test_non_socket: assertion failed (error == NULL): Exhausted all available authentication mechanisms (tried: EXTERNAL, DBUS_COOKIE_SHA1) (available: EXTERNAL, DBUS_COOKIE_SHA1) (g-io-error-quark, 0)
```

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

gio/tests/gdbus-non-socket.c

@@ -293,7 +293,7 @@ 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_func ("/gdbus/non-socket", test_non_socket);