itest: unify PSBT channel funding tests, add taproot

With this commit we unify three existing PSBT channel funding
integration tests into a single one with the goal of testing all three
cases with simple taproot channels as well.

itest/list_on_test.go

@@ -269,14 +269,6 @@ var allTestCases = []*lntest.TestCase{
 		Name:     "psbt channel funding",
 		TestFunc: testPsbtChanFunding,
 	},
-	{
-		Name:     "psbt channel funding external",
-		TestFunc: testPsbtChanFundingExternal,
-	},
-	{
-		Name:     "psbt channel funding single step",
-		TestFunc: testPsbtChanFundingSingleStep,
-	},
 	{
 		Name:     "sign psbt",
 		TestFunc: testSignPsbt,