lntest: add build flag for disabling txindex on bitcoind

We create a new build flag for running the bitcoind tests without the
txindex enabled. We don't want this to be the default so we use a
negated build flag.

lntest/bitcoind.go

@@ -1,4 +1,5 @@
 // +build bitcoind
+// +build !notxindex
 
 package lntest
 

lntest/bitcoind_notxindex.go

@@ -0,0 +1,22 @@
+// +build bitcoind
+// +build notxindex
+
+package lntest
+
+import (
+	"github.com/btcsuite/btcd/chaincfg"
+)
+
+// NewBackend starts a bitcoind node without the txindex enabled and returns a
+// BitoindBackendConfig for that node.
+func NewBackend(miner string, netParams *chaincfg.Params) (
+	*BitcoindBackendConfig, func() error, error) {
+
+	extraArgs := []string{
+		"-debug",
+		"-regtest",
+		"-disablewallet",
+	}
+
+	return newBackend(miner, netParams, extraArgs)
+}