Test generated-route valididty in fuzzing

[TheBlueMatt]

While our router fuzzer is pretty good at hitting internal
assertions in our pathfinder, it doesn't actually do anything to
check that the returned route is valid (or meets the requirements
given to the pathfinder).

Here we add some initial checks covering the feerates of the hops
taken in the returned route.

Currently fails because of https://github.com/lightning/bolts/issues/1249

[ldk-reviews-bot]

👋 Hi! This PR is now in draft status. I'll wait to assign reviewers until you mark it as ready for review. Just convert it out of draft status when you're ready for review!

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 89.13%. Comparing base (233aa39) to head (908c5bc).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #3728   +/-   ##
=======================================
  Coverage   89.13%   89.13%           
=======================================
  Files         157      157           
  Lines      123959   123968    +9     
  Branches   123959   123968    +9     
=======================================
+ Hits       110485   110497   +12     
- Misses      10791    10796    +5     
+ Partials     2683     2675    -8     

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