test: add test for gccgo comment lexing failure

http://gcc.gnu.org/PR61746

http://code.google.com/p/gofrontend/issues/detail?id=35

LGTM=crawshaw
R=golang-codereviews, crawshaw
CC=golang-codereviews
https://golang.org/cl/111980043

test/fixedbugs/bug486.go

+// compile
+
+// Copyright 2014 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.
+
+// The gccgo lexer had a bug handling nested comments.
+// http://gcc.gnu.org/PR61746
+// http://code.google.com/p/gofrontend/issues/detail?id=35
+
+package main
+
+/*// comment
+*/