Add test files for l3str module covering clear LuaTeX bugs

These are all 'not our fault' and can be expected to change when LuaTeX is
next updated 'in the wild'.


git-svn-id: http://www.latex-project.org/svnroot/experimental/trunk@4992 de43f980-851b-0410-b2f7-c40aca1f87e0