Improve label generation code.
Make `cat%' into a macro so `~a' etc get a proper name.
Improve `file' tests.

* Move sha1 test to the same place, to be run like the others.

* Unify tests for untar and unzip.

* Also improve them: test results instead of failing with errors.  Also,
  generate random text to archive, and use 0 for group+other permission
  bits (to avoid world writable results on an error).