net: race condition in TestTCPStress

[mikioh]

I overlooked this during go1.5 cycle.

go test -v -race -run=TestTCPStress
=== RUN   TestTCPStress
--- PASS: TestTCPStress (0.38s)
PASS
==================
WARNING: DATA RACE
Write by main goroutine:
  net.uninstallTestHooks()
      /go/src/net/main_unix_test.go:37 +0x6f
  sync.(*Once).Do()
      /go/src/sync/once.go:44 +0xf6
  net.TestMain()
      /go/src/net/main_test.go:52 +0x62
  main.main()
      net/_test/_testmain.go:436 +0x20a

Previous read by goroutine 7:
  net.(*netFD).destroy()
      /go/src/net/fd_unix.go:153 +0x52
  net.(*netFD).readUnlock()
      /go/src/net/fd_unix.go:187 +0x52
  net.(*netFD).accept()
      /go/src/net/fd_unix.go:436 +0x32e
  net.(*TCPListener).AcceptTCP()
      /go/src/net/tcpsock_posix.go:254 +0x76
  net.(*TCPListener).Accept()
      /go/src/net/tcpsock_posix.go:264 +0x4a
  net.TestTCPStress.func3()
      /go/src/net/tcp_test.go:545 +0x38

Goroutine 7 (finished) created at:
  net.TestTCPStress()
      /go/src/net/tcp_test.go:560 +0x308
  testing.tRunner()
      /go/src/testing/testing.go:467 +0xdc
==================
Socket statistical information:
(inet4, stream, default): opened=3, connected=2, listened=1, accepted=2, closed=5, openfailed=0, connectfailed=2, listenfailed=0, acceptfailed=2, closefailed=0

[gopherbot]

CL https://golang.org/cl/18111 mentions this issue.