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_zero_conf_test.go

@@ -753,7 +753,7 @@ func testPrivateUpdateAlias(ht *lntest.HarnessTest,
 // testOptionScidUpgrade tests that toggling the option-scid-alias feature bit
 // correctly upgrades existing channels.
 func testOptionScidUpgrade(ht *lntest.HarnessTest) {
-	bob := ht.Bob
+	bob := ht.NewNodeWithCoins("Bob", nil)
 
 	// Start carol with anchors only.
 	carolArgs := []string{