Rename l3experimental/l3str test file to avoid name clash

Since there is now an m3str001.lvt in l3kernel, it is probably best not to
also have one here.


git-svn-id: http://www.latex-project.org/svnroot/experimental/trunk@5015 de43f980-851b-0410-b2f7-c40aca1f87e0