cmd/fix: disable reflect test under race detector (very slow)

R=golang-dev, rsc
CC=golang-dev
https://golang.org/cl/6642045

src/cmd/fix/reflect_test.go

@@ -2,6 +2,9 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file
 
+// Too slow under race detector.
+// +build !race
+
 package main
 
 import (