Expose process_pending_update_add_htlcs in tests

[valentinewallace]

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

Backported to 0.2 for testing on 0.3

[ldk-reviews-bot]

I've assigned @jkczyz 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.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 88.87%. Comparing base (e2ca5ba) to head (8ec37c9).
⚠️ Report is 2 commits behind head on 0.2.

Additional details and impacted files

@@           Coverage Diff           @@
##              0.2    #4240   +/-   ##
=======================================
  Coverage   88.87%   88.87%           
=======================================
  Files         180      180           
  Lines      138000   138003    +3     
  Branches   138000   138003    +3     
=======================================
+ Hits       122642   122650    +8     
- Misses      12534    12540    +6     
+ Partials     2824     2813   -11     

Flag	Coverage Δ
fuzzing	20.84% <25.00%> (-0.59%)	⬇️
tests	88.71% <100.00%> (+<0.01%)	⬆️

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[TheBlueMatt]

Sure