libxml2: fix test data checksums

* 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:

  xmlts20080827.tar:      POSIX tar archive (GNU)
  xmlts20080827.tar.gz:   POSIX tar archive (GNU)

  -rw-r--r--  1 martin martin 5.7M Sep  1  2008 xmlts20080827.tar
  -rw-r--r--  1 martin martin 5.7M Sep  1  2008 xmlts20080827.tar.gz

  9b2c865aba66c6429ca301a7ef048d7eca2cdb7a9106184416710853c7b37d0d  xmlts20080827.tar
  9b2c865aba66c6429ca301a7ef048d7eca2cdb7a9106184416710853c7b37d0d  xmlts20080827.tar.gz
  96151685cec997e1f9f3387e3626d61e6284d4d6e66e0e440c209286c03e9cc7  /OE/build/downloads/xmlts20080827.tar.gz

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