gtestutils: Allow failing a test with a printf-style message

This allows a pattern like

    g_test_message ("cannot reticulate splines: %s", error->message);
    g_test_fail ();

to be replaced by the simpler

    g_test_fail_printf ("cannot reticulate splines: %s", error->message);

with the secondary benefit of making the message available to TAP
consumers as part of the "not ok" message.

Signed-off-by: Simon McVittie <smcv@collabora.com>

docs/reference/glib/glib-sections.txt

@@ -3569,6 +3569,7 @@ g_test_get_filename
 g_test_get_dir
 
 g_test_fail
+g_test_fail_printf
 g_test_skip
 g_test_skip_printf
 g_test_incomplete