net: data race in TestSocket{Conn,PacketConn}

[mikioh]

The tester goroutine should wait for writer goroutines to stop for uninstalling test hooks gracefully.

==================
WARNING: DATA RACE
Write by main goroutine:
  net.uninstallTestHooks()
      /go/src/net/main_unix_test.go:37 +0x73
  sync.(*Once).Do()
      /go/src/sync/once.go:44 +0xf9
  net.TestMain()
      /go/src/net/main_test.go:46 +0x65
  main.main()
      net/_test/_testmain.go:396 +0x20c

Previous read by goroutine 9:
  net.(*netFD).destroy()
      /go/src/net/fd_unix.go:136 +0x55
  net.(*netFD).writeUnlock()
      /go/src/net/fd_unix.go:186 +0x59
  net.(*netFD).sendTo()
      /go/src/net/fd_unix.go:410 +0x376
  net.(*socketFile).WriteTo()
      /go/src/net/file_unix.go:179 +0x202
  net.TestSocketPacketConn.func1()
      /go/src/net/file_linux_test.go:73 +0x332

Goroutine 9 (finished) created at:
  net.TestSocketPacketConn()
      /go/src/net/file_linux_test.go:77 +0x66c
  testing.tRunner()
      /go/src/testing/testing.go:454 +0xfb
==================

[gopherbot]

CL https://golang.org/cl/10194 mentions this issue.