tmetadata.py: chmod tree before deletion to fix NetBSD test failure.

Apparently NetBSD won't let you recursively delete a parent directory
containing a 000 subdirectory, so "chmod -R u+rwX" the tree first.

Signed-off-by: Thomas Klausner <tk@giga.or.at>
[rlb@defaultvalue.org: adjust commit message.]
Reviewed-by: Rob Browning <rlb@defaultvalue.org>