l3build: Run "check" as part of CTAN process

Probably there need to be a few internal changes to make this efficient,
but it does work as currently set up.


git-svn-id: http://www.latex-project.org/svnroot/experimental/trunk@5125 de43f980-851b-0410-b2f7-c40aca1f87e0
l3build: Ensure "clean" really tidies up

Without this, some odd things can result from doing "ctan".


git-svn-id: http://www.latex-project.org/svnroot/experimental/trunk@5126 de43f980-851b-0410-b2f7-c40aca1f87e0