lntest: fix commitment TX not found in mempool error

This commit fixes an old flake in the neutrino anchor output tests. It
turns out that sometimes with Neutrino we don't have enough UTXOs in our
wallet to sweep both the local and remote anchor. This is very likely a
timing issue, we need to give the wallet more time to catch up with the
chain and process all transactions to find unspent outputs.
We address this two-fold: We add an additional UTXO to Alice. And then
we also make sure we detect both UTXOs properly after restarting.

lntest/harness.go

@@ -772,7 +772,15 @@ func (n *NetworkHarness) RestartNode(node *HarnessNode, callback func() error,
 		unlockReq.RecoveryWindow = 1000
 	}
 
-	return node.Unlock(context.Background(), unlockReq)
+	if err := node.Unlock(context.Background(), unlockReq); err != nil {
+		return err
+	}
+
+	// Give the node some time to catch up with the chain before we continue
+	// with the tests.
+	ctxc, done := context.WithTimeout(context.Background(), DefaultTimeout)
+	defer done()
+	return node.WaitForBlockchainSync(ctxc)
 }
 
 // RestartNodeNoUnlock attempts to restart a lightning node by shutting it down