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_sweep_test.go

@@ -1558,7 +1558,7 @@ func testSweepCommitOutputAndAnchor(ht *lntest.HarnessTest) {
 // CPFP, then RBF. Along the way, we check the `BumpFee` can properly update
 // the fee function used by supplying new params.
 func testBumpFee(ht *lntest.HarnessTest) {
-	alice := ht.Alice
+	alice := ht.NewNodeWithCoins("Alice", nil)
 
 	runBumpFee(ht, alice)
 }