lntest+itest: expore method createSimpleNetwork

So it can be used via `ht.CreateSimpleNetwork`.

itest/lnd_misc_test.go

@@ -1421,9 +1421,10 @@ func testSendSelectedCoinsChannelReserve(ht *lntest.HarnessTest) {
 	// Create a two-hop network: Alice -> Bob.
 	//
 	// NOTE: Alice will have one UTXO after the funding.
-	_, nodes := createSimpleNetwork(
-		ht, []string{"--protocol.anchors"}, 2,
-		lntest.OpenChannelParams{
+	cfg := []string{"--protocol.anchors"}
+	cfgs := [][]string{cfg, cfg}
+	_, nodes := ht.CreateSimpleNetwork(
+		cfgs, lntest.OpenChannelParams{
 			Amt: chanAmt,
 		},
 	)