Add regrestion test ensuring we avoid name mangling in user specified checkout directories

[See original issue on GitLab](https://gitlab.com/BuildStream/buildstream/-/issues/1363)
In GitLab by [[Gitlab user @willsalmon]](https://gitlab.com/willsalmon) on Jul 13, 2020, 08:36

## Summary

The fix in https://gitlab.com/BuildStream/buildstream/-/merge_requests/1992 is very simple and allow our next dev snapshot to work with projects that did work with bst1.4. But longer term the comment https://gitlab.com/BuildStream/buildstream/-/merge_requests/1992#note_377505495 seems very sensible.

With https://gitlab.com/BuildStream/buildstream/-/merge_requests/1992 merged we should regression test it.

Start with GitLab merge request 1992 and its discussion to identify the checkout-directory behavior that was fixed. Add a regression test for user-specified checkout directories whose names must not be mangled, then run the relevant test suite to confirm the preserved names.

Written by the indexing model from the issue text.