runtime/pprof: data race in test

[dvyukov]

Race builders say:

WARNING: DATA RACE
Write by goroutine 39:
  os.(*file).close()
      /tmp/buildlet-scatch779161213/go/src/os/file_unix.go:121 +0x1d1
  os.(*File).Close()
      /tmp/buildlet-scatch779161213/go/src/os/file_unix.go:110 +0x8b
  runtime/pprof_test.TestTraceSymbolize()
      /tmp/buildlet-scatch779161213/go/src/runtime/pprof/trace_stack_test.go:282 +0x2812
  testing.tRunner()
      /tmp/buildlet-scatch779161213/go/src/testing/testing.go:452 +0xfc

Previous read by goroutine 51:
  os.(*File).read()
      /tmp/buildlet-scatch779161213/go/src/os/file_unix.go:203 +0x98
  os.(*File).Read()
      /tmp/buildlet-scatch779161213/go/src/os/file.go:95 +0xbf
  runtime/pprof_test.TestTraceSymbolize.func11()
      /tmp/buildlet-scatch779161213/go/src/runtime/pprof/trace_stack_test.go:100 +0x98

Goroutine 39 (running) created at:
  testing.RunTests()
      /tmp/buildlet-scatch779161213/go/src/testing/testing.go:560 +0xc84
  testing.(*M).Run()
      /tmp/buildlet-scatch779161213/go/src/testing/testing.go:490 +0xed
  main.main()
      /tmp/go-build411804972/runtime/pprof/_test/_testmain.go:82 +0x28c

Goroutine 51 (finished) created at:
  runtime/pprof_test.TestTraceSymbolize()
      /tmp/buildlet-scatch779161213/go/src/runtime/pprof/trace_stack_test.go:101 +0x8d2
  testing.tRunner()
      /tmp/buildlet-scatch779161213/go/src/testing/testing.go:452 +0xfc

http://build.golang.org/log/302f686e6f27540f10de4242d222cb5761677eda