gutils: Add a private API to unset the cached temporary directory

We may need to avoid using a cached temp directory for testing purposes,
so let's provide an internal API to perform such task.

This implies removing GOnce and going with mutex-based version, but
that's still using atomic logic in most unix implementations anyways.

tools/glib.supp

@@ -840,6 +840,19 @@
 	fun:g_set_user_dirs
 }
 
+# _g_unset_cached_tmp_dir() deliberately leaks the previous cached g_get_tmp_dir() values.
+# These will not all be reachable on exit.
+{
+	g_get_tmp_dir
+	Memcheck:Leak
+	match-leak-kinds:definite,reachable
+	fun:malloc
+	...
+	fun:g_get_tmp_dir
+	...
+	fun:g_test_init
+}
+
 # g_get_system_data_dirs() caches a one-time allocation
 {
 	g_get_system_data_dirs