Add undo tests

[rewin123]

~85% code coverage for undo crate

[codecov]

Codecov Report

Attention: Patch coverage is 98.81657% with 2 lines in your changes are missing coverage. Please review.

Project coverage is 22.03%. Comparing base (b33cd11) to head (4c65bb9).
Report is 30 commits behind head on main.

Files	Patch %	Lines
crates/undo/src/lib.rs	50.00%	1 Missing ⚠️
crates/undo/src/tests.rs	99.40%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #217      +/-   ##
==========================================
+ Coverage   12.74%   22.03%   +9.28%     
==========================================
  Files          54       54              
  Lines        7085     7503     +418     
==========================================
+ Hits          903     1653     +750     
+ Misses       6182     5850     -332     

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