itest: test "lnd --debuglevel=show" command

Tests that "lnd --debuglevel=show" command works and prints the list of
supported subsystems.

itest/list_on_test.go

@@ -702,4 +702,8 @@ var allTestCases = []*lntest.TestCase{
 		Name:     "send to route failed htlc timeout",
 		TestFunc: testSendToRouteFailHTLCTimeout,
 	},
+	{
+		Name:     "debuglevel show",
+		TestFunc: testDebuglevelShow,
+	},
 }