Fix channel reserve calculation on the sending side

[TheBlueMatt]

As the variable name implies holder_selected_chan_reserve_msat is
intended to be in millisatoshis, but is instead calculated in
satoshis.

We fix that error here and update the relevant tests to more
accurately calculate the expected reserve value and test both
success and failure cases.

Bug discovered by chanmon_consistency fuzz target.

[codecov]

Codecov Report

Merging #998 (306e9a5) into main (afae12e) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main     #998   +/-   ##
=======================================
  Coverage   90.78%   90.78%           
=======================================
  Files          60       60           
  Lines       30914    30929   +15     
=======================================
+ Hits        28064    28079   +15     
  Misses       2850     2850           

Impacted Files	Coverage Δ
lightning/src/ln/channel.rs	88.66% <100.00%> (ø)
lightning/src/ln/functional_tests.rs	97.41% <100.00%> (+<0.01%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update afae12e...306e9a5. Read the comment docs.

[valentinewallace]

Nice catch!

[TheBlueMatt]

Nice catch!

Thank the gods of honggfuzz.

[ariard]

ACK 306e9a5

I've played a bit with the modified tests and checked other invocations of get_holder_selected_channel_reserve_satoshis()