test: add test for invalid nil == nil

R=gri
CC=golang-dev
https://golang.org/cl/6868059

test/fixedbugs/issue4283.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 4283: nil == nil can't be done as the type is unknown.
+
+package p
+
+func F1() bool {
+	return nil == nil	// ERROR "invalid"
+}
+
+func F2() bool {
+	return nil != nil	// ERROR "invalid"
+}