Do not panic on early tx broadcasts in fuzzing

[TheBlueMatt]

If the user broadcasts a funding transaction before the
counterparty provides a funding_signed we will panic in
check_get_channel_ready. This is expected - the user did
something which may lead to loss of funds, and we really need to
let them know.

However, the fuzzer can do this and we shouldn't treat it as a bug,
its a totally expected panic. Thus, we disable the panic in fuzz.

Thanks to Chaincode for providing fuzzing resources which managed
to hit this panic.

[codecov-commenter]

Codecov Report

Merging #1513 (47045b0) into main (0b77008) will decrease coverage by 0.05%.
The diff coverage is 33.33%.

@@            Coverage Diff             @@
##             main    #1513      +/-   ##
==========================================
- Coverage   91.01%   90.96%   -0.06%     
==========================================
  Files          80       80              
  Lines       43409    43410       +1     
  Branches    43409    43410       +1     
==========================================
- Hits        39509    39486      -23     
- Misses       3900     3924      +24     

Impacted Files	Coverage Δ
lightning/src/ln/channel.rs	88.57% <33.33%> (-0.03%)	⬇️
lightning/src/chain/onchaintx.rs	93.98% <0.00%> (-0.93%)	⬇️
lightning/src/util/events.rs	41.66% <0.00%> (-0.33%)	⬇️
lightning/src/ln/functional_tests.rs	96.95% <0.00%> (-0.24%)	⬇️
lightning/src/ln/payment_tests.rs	99.06% <0.00%> (-0.19%)	⬇️
lightning-invoice/src/utils.rs	96.63% <0.00%> (-0.15%)	⬇️

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