test: add a few more unit tests

[JoshuaKGoldberg]

PR Checklist

• Addresses an existing open issue: continues on 🧪 Tests: Add tests for remaining functions in src/nodes #7, 🧪 Tests: Add tests for remaining functions in src/types #8
• That issue was marked as status: accepting prs
• Steps in CONTRIBUTING.md were taken

Overview

It's been bugging me that this package now has >2.5 million downloads but only has 46% coverage. Which Codecov shows as a red color in the badge. 😡

No end user impacts here. Just added test coverage.

[codecov]

Codecov Report

Merging #262 (0fd9d45) into main (f039b2b) will increase coverage by 3.69%.
Report is 1 commits behind head on main.
The diff coverage is n/a.

❗ Current head 0fd9d45 differs from pull request most recent head 6c95baf. Consider uploading reports for the commit 6c95baf to get more accurate results

@@            Coverage Diff             @@
##             main     #262      +/-   ##
==========================================
+ Coverage   46.30%   50.00%   +3.69%     
==========================================
  Files          20       20              
  Lines         460      460              
  Branches      181      181              
==========================================
+ Hits          213      230      +17     
+ Misses        218      203      -15     
+ Partials       29       27       -2     

Flag	Coverage Δ
4.3.5	49.34% <ø> (+3.47%)	⬆️
4.6.4	49.56% <ø> (+3.47%)	⬆️
4.9.5	50.00% <ø> (+3.69%)	⬆️
latest	49.78% <ø> (+3.69%)	⬆️

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see 5 files with indirect coverage changes

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