add l3build.dtx to produce l3build.tex

* not sure about the name but l3build.tex would be the new regression-test.tex

* probably good idea to add the lua script to the dtx when it stabilises

* and the dtx will document the whole thing of course :)

git-svn-id: http://www.latex-project.org/svnroot/experimental/trunk@4994 de43f980-851b-0410-b2f7-c40aca1f87e0