itest: add new RBF coop close itest

The itest has both sides try to close multiple times, each time with
increasing fee rates. We also test the reconnection case, bad RBF
updates, and instances where the local party can't actually pay for
fees.

itest/list_on_test.go

@@ -678,6 +678,10 @@ var allTestCases = []*lntest.TestCase{
 		Name:     "access perm",
 		TestFunc: testAccessPerm,
 	},
+	{
+		Name:     "rbf coop close",
+		TestFunc: testCoopCloseRbf,
+	},
 }
 
 // appendPrefixed is used to add a prefix to each test name in the subtests