Allow clippy's new assertions-on-constants lint

[TheBlueMatt]

This is really dumb, assert!(cfg!(fuzzing)) is a perfectly reasonable thing to write!

[ldk-reviews-bot]

I've assigned @joostjager 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.

[TheBlueMatt]

Linting passes now.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 89.37%. Comparing base (765e43d) to head (6ff720b).
⚠️ Report is 24 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff            @@
##             main    #4277    +/-   ##
========================================
  Coverage   89.36%   89.37%            
========================================
  Files         180      180            
  Lines      139366   139599   +233     
  Branches   139366   139599   +233     
========================================
+ Hits       124547   124767   +220     
- Misses      12227    12248    +21     
+ Partials     2592     2584     -8     

Flag	Coverage Δ
fuzzing	35.14% <ø> (-0.03%)	⬇️
tests	88.70% <ø> (+<0.01%)	⬆️

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