Zero fee commitment test vectors

[carlaKC]

Needs some serious cleaning up, so please don't review yet!

Opening up early because I'll be out till the 10th, want to have a passing version of these up in the meantime.

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[codecov]

Codecov Report

❌ Patch coverage is 42.85714% with 12 lines in your changes missing coverage. Please review.
✅ Project coverage is 89.31%. Comparing base (ec0b969) to head (c9bd30a).

Files with missing lines	Patch %	Lines
lightning/src/util/test_utils.rs	42.85%	12 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4202      +/-   ##
==========================================
- Coverage   89.34%   89.31%   -0.03%     
==========================================
  Files         180      180              
  Lines      138166   138178      +12     
  Branches   138166   138178      +12     
==========================================
- Hits       123438   123420      -18     
- Misses      12123    12153      +30     
  Partials     2605     2605              

Flag	Coverage Δ
fuzzing	35.81% <0.00%> (-0.02%)	⬇️
tests	88.69% <42.85%> (-0.03%)	⬇️

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[carlaKC]

Rebased + cleaned this up - ready for a look now. Two notes for reviewers:

• There are quite a few thin from_hex helpers, IMO they help with readability quite a lot so are worth having.
• Didn't move this into its own file because it would require making a whole bunch of things pub(crate), happy to do so if there's strong preferences!