Add millisatoshi truncation 0FC test vectors from the spec

[tankyleo]

Millisatoshi truncation also contributes to the value of the anchor output on a zero-fee commitment transaction.

[ldk-reviews-bot]

👋 Thanks for assigning @carlaKC as a reviewer!
I'll wait for their review and will help manage the review process.
Once they submit their review, I'll check if a second reviewer would be helpful.

[carlaKC]

LGTM!

IMO can be merged with one review ✅

[ldk-reviews-bot]

👋 The first review has been submitted!

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 89.31%. Comparing base (6d9c676) to head (fd12205).
⚠️ Report is 5 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4237      +/-   ##
==========================================
- Coverage   89.32%   89.31%   -0.02%     
==========================================
  Files         180      180              
  Lines      138641   138641              
  Branches   138641   138641              
==========================================
- Hits       123844   123826      -18     
- Misses      12174    12193      +19     
+ Partials     2623     2622       -1     

Flag	Coverage Δ
fuzzing	35.97% <ø> (ø)
tests	88.68% <ø> (-0.02%)	⬇️

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