Add several tests for different formatting methods

[rich-iannone]

Quite a few of the formatting methods had very low test coverage so this PR provides some testing for the following:

• [x] fmt_roman()
• [x] fmt_bytes()
• [x] fmt_time()
• [x] fmt_datetime()
• [x] fmt_markdown()
• [x] fmt_nanoplot()

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 79.92%. Comparing base (d4178e0) to head (c9ff081).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #260 +/- ## ========================================== + Coverage 75.26% 79.92% +4.66% ========================================== Files 41 41 Lines 4180 4180 ========================================== + Hits 3146 3341 +195 + Misses 1034 839 -195 ```

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[rich-iannone]

I believe all the comments have been addressed through the changes we made during code pairing. I handled a merge conflict and reformatted to satisfy the pre-commit error.