chore: add tests for code segments

[JoshuaKGoldberg]

PR Checklist

• Addresses an existing open issue: fixes 🛠 Tooling: Add unit tests for code segments #19
• That issue was marked as status: accepting prs
• Steps in CONTRIBUTING.md were taken

Overview

Fills in most of the missing test coverage.

[codecov]

Codecov Report

Merging #20 (d17a20f) into main (1baeec6) will increase coverage by 30.30%.
Report is 1 commits behind head on main.
The diff coverage is n/a.

@@             Coverage Diff             @@
##             main      #20       +/-   ##
===========================================
+ Coverage   56.06%   86.36%   +30.30%     
===========================================
  Files           2        2               
  Lines         132      132               
  Branches       13       25       +12     
===========================================
+ Hits           74      114       +40     
+ Misses         58       18       -40     

see 1 file with indirect coverage changes

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