Fix unish_file_delete_recursive() out of sync with drush_delete_dir()

`unish_file_delete_recursive()` claims that it is the same code as `drush_delete_dir()`, but it's actually quite different. Most notably, it follows symlinks by default instead of unlinking them, which is unexpectedly destructive. This pull request gets the two functions back in sync... but as I indicated in the comments, I think this sort of duplication is unmaintainable. Is there a reason not to just include includes/filesystem.inc in tests/drush_testcase.inc and use `drush_delete_dir()` itself? If there is, can the duplication be eliminated some other way?