itest: add disconnect test

Add a connect/disconnect test when the peers both started up
with the rbf coop close feature.

itest/list_on_test.go

@@ -699,6 +699,10 @@ var allTestCases = []*lntest.TestCase{
 		Name:     "rbf coop close",
 		TestFunc: testCoopCloseRbf,
 	},
+	{
+		Name:     "rbf coop close disconnect",
+		TestFunc: testRBFCoopCloseDisconnect,
+	},
 	{
 		Name:     "bump fee low budget",
 		TestFunc: testBumpFeeLowBudget,