lnwallet/interface_test: extend publish timeout

This commit extends the amount of time we wait
for transaction to enter the mempool from
10 to 30 seconds. The wallet's interface tests
seem to be particularly slow when run with the
race flag, a problem which is only exacerbated
by the slowness of travis.

With 10s and the race flag, I was able to repro
the issues locally fairly consistently.

lnwallet/interface_test.go

@@ -1841,7 +1841,7 @@ func waitForMempoolTx(r *rpctest.Harness, txid *chainhash.Hash) error {
 	var found bool
 	var tx *btcutil.Tx
 	var err error
-	timeout := time.After(10 * time.Second)
+	timeout := time.After(30 * time.Second)
 	for !found {
 		// Do a short wait
 		select {