libxml2: upgrade test data from 20080827 to 20130923

* and switch from tar.gz to tar, because the tar.gz archives upstream are regular tar as well now
  https://www.w3.org/XML/Test/

 still has 3 separate URLs for .zip, .tar
  and .tar.gz, but both tar links return the same file:

  xmlts20130923.tar:      POSIX tar archive (GNU)
  xmlts20130923.tar.gz:   POSIX tar archive (GNU)
  xmlts20130923.zip:      Zip archive data, at least v1.0 to extract, compression method=store

  -rw-r--r--  1 martin martin 5.7M Sep 23  2013 xmlts20130923.tar
  -rw-r--r--  1 martin martin 5.7M Sep 23  2013 xmlts20130923.tar.gz
  -rw-r--r--  1 martin martin 1.6M Sep 23  2013 xmlts20130923.zip

  c6b2d42ee50b8b236e711a97d68e6c4b5c8d83e69a2be4722379f08702ea7273  xmlts20130923.tar
  c6b2d42ee50b8b236e711a97d68e6c4b5c8d83e69a2be4722379f08702ea7273  xmlts20130923.tar.gz
  f9510b3532926e1b4c2e54855b021e4b8a66ec98a5337dcf4ff07e8a41968deb  xmlts20130923.zip

Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>