test: decrease amount sent in multi-hop error integration test

In this commit, we increase the initial amount sent in the multi-hop
error integration test. We must do this now as the Bandwidth() function
now takes into account the total fee paid within the commitment
transaction. This caused an earlier send to fail instead of the once
following send.

To fix this, we simply send less in the initial test case.

lnd_test.go

@@ -3019,7 +3019,7 @@ out:
 	// Alice's side, leaving on 10k satoshis of available balance for bob.
 	// There's a max payment amount, so we'll have to do this
 	// incrementally.
-	amtToSend := int64(chanAmt) - 10000
+	amtToSend := int64(chanAmt) - 20000
 	amtSent := int64(0)
 	for amtSent != amtToSend {
 		// We'll send in chunks of the max payment amount. If we're