test: add test for issue 4468 (go/defer expr may not be parenthesized)

R=gri
CC=golang-dev
https://golang.org/cl/6870053

test/fixedbugs/issue4468.go

+// errorcheck
+
+// Copyright 2012 The Go Authors.  All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Issue 4468: go/defer calls may not be parenthesized.
+
+package p
+
+func F() {
+	go (F())	// ERROR "must be function call"
+	defer (F())	// ERROR "must be function call"
+}