add unit tests to cleanup.R

[awasyn]

Add unit tests to cleanup

• Dependent on : https://github.com/JRaviLab/MolEvolvR/pull/75 (relies on testtthat already added)

• Task: https://github.com/JRaviLab/MolEvolvR/issues/29

• [x] Enhancement (unit tests).

Checklist

• [x] I have read and followed the CONTRIBUTING.md guidelines.
• [x] I have searched for existing content to ensure this is not a duplicate.
• [x] I have performed a self-review of these additions (including spelling, grammar, and related).
• [x] I have added comments to my code to help provide understanding.
• [x] I have added a test which covers the code changes found within this PR.
• [x] I have deleted all non-relevant text in this pull request template.
• [x] Reviewer assignment: Tag a relevant team member to review and approve the changes.

@the-mayer