Test preimage extraction from mempool

[t-bast]

Instead of waiting for htlc-success txs to be confirmed, eclair also looks at mempool txs to detect preimages as soon as possible.

This has been the case for a very long time, but our integration tests didn't showcase this correctly.

[pm47]

We should test both cases: a tx that we haven't seen in the mempool may appear in a block.

[t-bast]

We should test both cases: a tx that we haven't seen in the mempool may appear in a block.

Indeed. But I think we won't be able to test that in the IntegrationSpec.
I'll check what we already have in the non E2E tests, we probably already test that.

[t-bast]

I added tests (and test helpers) to ZmqWatcher in 09a308d. The ClosingStateSpec already has tests for preimage extraction from WatchEventSpents so we should be covering most of it now.

[codecov-io]

Codecov Report

Merging #1387 into master will increase coverage by 0.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #1387      +/-   ##
==========================================
+ Coverage   86.68%   86.69%   +0.01%     
==========================================
  Files         123      123              
  Lines        9456     9456              
  Branches      388      388              
==========================================
+ Hits         8197     8198       +1     
+ Misses       1259     1258       -1     

Impacted Files	Coverage Δ
...q/eclair/blockchain/electrum/ElectrumWatcher.scala	53.60% <0.00%> (-1.61%)	⬇️
...in/scala/fr/acinq/eclair/channel/Commitments.scala	90.96% <0.00%> (-0.34%)	⬇️
...clair/blockchain/electrum/ElectrumClientPool.scala	82.79% <0.00%> (+4.30%)	⬆️