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_taproot_test.go

@@ -49,7 +49,7 @@ var (
 // testTaproot ensures that the daemon can send to and spend from taproot (p2tr)
 // outputs.
 func testTaproot(ht *lntest.HarnessTest) {
-	alice := ht.Alice
+	alice := ht.NewNode("Alice", nil)
 
 	testTaprootSendCoinsKeySpendBip86(ht, alice)
 	testTaprootComputeInputScriptKeySpendBip86(ht, alice)