Clean up properly at distclean

Remove test data and other files on distclean, and stop shipping the
generated ChangeLog too. The reason for not shipping the ChangeLog is
due to the following issues:

 * It's not useful, NEWS covers the user-visible changes in a much more
   digestible format, and the git log is available to everyone who wants
   or needs the more fine grained log.
 * The ChangeLog can only be regenerated from a git checkout, which
   makes it hard to remove it when doing distclean.
 * Not removing it at distclean time makes distcheck fail, and working
   around that is more work than the benefit of having ChangeLog at all.

Signed-off-by: Gergely Nagy <algernon@balabit.hu>