Subtree consistency tests for subtrees starting at 0

[phbnf]

This PR adds subtree consistency tests for subtree starting at 0. These are the same tests that are used for consistency tests. We'll add more tests with inner subtrees later.

Towards #225.

[codecov]

Codecov Report

❌ Patch coverage is 0% with 80 lines in your changes missing coverage. Please review.
✅ Project coverage is 47.16%. Comparing base (b0ba6a7) to head (016b2df).

Files with missing lines	Patch %	Lines
cmd/proofgen/main.go	0.00%	80 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #247      +/-   ##
==========================================
- Coverage   48.99%   47.16%   -1.83%     
==========================================
  Files           8        8              
  Lines         845      918      +73     
==========================================
+ Hits          414      433      +19     
- Misses        422      476      +54     
  Partials        9        9              

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