itest: test second-level HTLC revocation in breach scenario

Add a custom-channel breach integration test that exercises revoked HTLCs which
advance to second level before the honest party comes back online and sweeps
them.

Keep the default scenario small for normal suite runtime, but structure the
itest so it can be scaled up to stress the same recovery path. Higher-count
variants need additional setup discipline: the pre-breach rebalance must scale
with the requested number of in-flight HTLCs, the post-settlement keysend must
wait until HTLC cleanup has actually completed, and justice mining must accept
the variable set of justice transactions published once more HTLCs reach second
level.

Make the breach test configurable through TAPD_BREACH_HODL_INVOICES_PER_SIDE
while keeping the default at two HTLCs per side.

Author: GeorgeTsagk

itest/custom_channels/assertions.go

@@ -367,8 +367,8 @@ func waitForNTxsInMempool(m *miner.HarnessMiner, n int,
 	}
 }
 
-// waitForAtLeastNTxsInMempool polls until finding at least n transactions
-// in the miner's mempool, returning all txids present.
+// waitForAtLeastNTxsInMempool polls until finding at least n
+// transactions in the miner's mempool, returning all txids present.
 func waitForAtLeastNTxsInMempool(m *miner.HarnessMiner, n int,
 	timeout time.Duration) ([]*chainhash.Hash, error) {
 
@@ -388,7 +388,8 @@ func waitForAtLeastNTxsInMempool(m *miner.HarnessMiner, n int,
 
 			if len(mempool) >= n {
 				result := make(
-					[]*chainhash.Hash, len(mempool),
+					[]*chainhash.Hash,
+					len(mempool),
 				)
 				for i := range mempool {
 					result[i] = &mempool[i]
@@ -399,6 +400,14 @@ func waitForAtLeastNTxsInMempool(m *miner.HarnessMiner, n int,
 	}
 }
 
+// waitForNonEmptyMempool polls until the mempool is non-empty, then
+// returns a snapshot of its current contents.
+func waitForNonEmptyMempool(m *miner.HarnessMiner,
+	timeout time.Duration) ([]*chainhash.Hash, error) {
+
+	return waitForAtLeastNTxsInMempool(m, 1, timeout)
+}
+
 // assertTxInBlock asserts that a transaction with the given hash is included
 // in the block.
 func assertTxInBlock(t *ccHarnessTest, block *wire.MsgBlock,