Expose process_pending_update_add_htlcs in tests

[valentinewallace]

Useful for upgrade/downgrade testing in the lightning-tests module, which cannot access internal methods.

Addresses #4240 (comment)

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 89.31%. Comparing base (3e186bc) to head (aa6a64b).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4253      +/-   ##
==========================================
- Coverage   89.31%   89.31%   -0.01%     
==========================================
  Files         180      180              
  Lines      138966   138969       +3     
  Branches   138966   138969       +3     
==========================================
- Hits       124121   124115       -6     
- Misses      12218    12232      +14     
+ Partials     2627     2622       -5     

Flag	Coverage Δ
fuzzing	35.97% <25.00%> (-0.01%)	⬇️
tests	88.67% <100.00%> (-0.01%)	⬇️

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