l3build: Clean up ctan/tds dirs when making "ctan" target

This is a proper approach to the fact that there is no "clean" step implied
in the other targets. As such, the top-level build script no longer needs
to have a one-off clean-up step in it.


git-svn-id: http://www.latex-project.org/svnroot/experimental/trunk@5243 de43f980-851b-0410-b2f7-c40aca1f87e0
lbuild: testsupdir => testsuppdir

Consistent with other 'supp' usage.


git-svn-id: http://www.latex-project.org/svnroot/experimental/trunk@5244 de43f980-851b-0410-b2f7-c40aca1f87e0