Fix main check function

Two things:
 - l3build has no tests
 - file listing should check dir exists, as this gives an easy way to
   avoid trying to access non-existent files


git-svn-id: http://www.latex-project.org/svnroot/experimental/trunk@5042 de43f980-851b-0410-b2f7-c40aca1f87e0