Use \tl_show:n/\cs_show:N for displaying output in xparse tests

This removes most engine-dependent differences. With TL2014, there are two
remaining, a box one with XeTeX and one from teh TeX error deliberately
generated in xparse002, where LuaTeX truncates differently from pdfTeX.


git-svn-id: http://www.latex-project.org/svnroot/experimental/trunk@4931 de43f980-851b-0410-b2f7-c40aca1f87e0