Add unit tests to CHANGED-pre-msa-tree

Dependent on: https://github.com/JRaviLab/MolEvolvR/pull/75
Task: https://github.com/JRaviLab/MolEvolvR/issues/29

What kind of change(s) are included?
[x] Enhancement (adds 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