mark utim_from_header as TESTING

This is really in for a long time but due to recent changes and because it was
undocumented, mark it non-stable.

Signed-off-by: Nicolas Sebrecht <nicolas.s-dev@laposte.net>

offlineimap.conf

@@ -468,6 +468,7 @@ localfolders = ~/Test
 # file/message content.
 #
 # If enabled, this forbid the -q (quick mode) CLI option to work correctly.
+# This option is still "TESTING" feature.
 #
 # Default: no.
 #