Add safer method to remove hywiki-folder

[matsl]

What

Add safer method to remove hywiki-folder.

Why

There is a small risk that the deletion of a test hywiki-directory
could touch a users real hywiki-directory. This PR creates a function
in hywiki-tests that eliminates this.

It works like this:

• The cache file is checked if it is the only file in the
  directory. If it is, it is removed.
• The whole folder is then removed. This step only works if the
  directory is empty and is ensured by delete-directory that does not
  delete directories with files.

Bonus is that hy-delete-dir-and-buffer now is reverted to its original
and does not know anything about hywiki caches.