[WIP, POC] Tests for Interactive TX

[optout21]

Based on: #2419
W-i-P, submitted to see tests & coverage.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 76.21359% with 98 lines in your changes missing coverage. Please review.
✅ Project coverage is 90.50%. Comparing base (61d896d) to head (deca874).
⚠️ Report is 5471 commits behind head on main.

Files with missing lines	Patch %	Lines
lightning/src/ln/interactivetxs.rs	76.84%	94 Missing ⚠️
lightning/src/ln/msgs.rs	0.00%	4 Missing ⚠️

Additional details and impacted files

@@ Coverage Diff @@## main #2541 +/- ##
==========================================
- Coverage 90.59% 90.50% -0.09% 
==========================================
Files 110 111 +1 Lines 57410 57917 +507 Branches 57410 57917 +507 ==========================================
+ Hits 52008 52416 +408 - Misses 5402 5501 +99 

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[ariard]

Don’t hesitate to propose tests if they can be generalized out of LDK as test vectors addendum for lightning/bolts#851 There is a matrix of test cases with channel type, and I’m not even sure dual funding / splicing are fleshed enough on this.

[optout21]

Patch coverage 76% -- with only a handful of tests and zero negative tests it seems to-good-to-be-true

[optout21]

Closing for now, interactive TX code is still work-in-progress