Run ocaml tests in isolated dirs, ensure proper stage cleanup

[serban-nicusor-toptal]

Submission Checklist

• [ ] Run unit tests
• Documentation
  • [ ] If a user-facing facing change was made, the documentation PR is here:
  • [x] OR, no user-facing changes were made

Release notes

[CI] A small issue where the files on disk are not cleaned up after ocaml testing stages has been found and fixed with this PR. Thanks @WardBrian for reporting it.

Copyright and Licensing

By submitting this pull request, the copyright holder is agreeing to license the submitted work under the BSD 3-clause license (https://opensource.org/licenses/BSD-3-Clause)