itest: remove the use of standby nodes

This commit removes the usage of the standby nodes and uses
`CreateSimpleNetwork` when applicable. Also introduces a helper method
`NewNodeWithCoins` to quickly start a node with funds.

itest/lnd_htlc_test.go

@@ -14,7 +14,7 @@ import (
 func testLookupHtlcResolution(ht *lntest.HarnessTest) {
 	const chanAmt = btcutil.Amount(1000000)
 
-	alice := ht.Alice
+	alice := ht.NewNodeWithCoins("Alice", nil)
 	carol := ht.NewNode("Carol", []string{
 		"--store-final-htlc-resolutions",
 	})