test(TaxForms): add fields snapshots

[Betree]

This will:

1. Provide an easy way to review the mappings
2. Help us detect regressions when updating fields
3. Assist with form upgrades when they get updated by the IRS

[codecov-commenter]

Codecov Report

❌ Patch coverage is 95.23810% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 60.40%. Comparing base (97e4e55) to head (2456f2a).
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
server/lib/tax-forms/utils.ts	95.23%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1172      +/-   ##
==========================================
+ Coverage   59.09%   60.40%   +1.30%     
==========================================
  Files          35       35              
  Lines        1017     1038      +21     
  Branches      345      351       +6     
==========================================
+ Hits          601      627      +26     
+ Misses        346      341       -5     
  Partials       70       70              

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