merge unstow_orig.t into unstow.t and fix unstowing logic

There was a ton of duplication which is not maintainable, so refactor
everything into a single test which still covers the differences.

This in turn revealed some issues in the unstowing logic:

- We shouldn't conflict if we find a file which isn't a link or a
  directory; we can just skip over it.

- Unstowing with `--dotfiles` was using the wrong variable to obtain
  the package path, and as a result having to perform an unnecessary
  call to `adjust_dotfile()`.

So fix those at the same time.

MANIFEST.SKIP

@@ -83,7 +83,7 @@
 ^doc/HOWTO-RELEASE$
 
 # Avoid test files
-tmp-testing-trees
+tmp-testing-trees*
 ^.coveralls.yml
 ^.github/workflows/
 ^.travis.yml