Revert "os: remove sleep in windows Process.Wait"

This reverts CL 145221 (commit 5c359736)

Reason for revert: breaks the build occasionally.

Updates #23171
Updates #25965

Change-Id: Ie1e3c76ab9bcd8d28b6118440b5f80c76f9b1852
Reviewed-on: https://go-review.googlesource.com/c/148957
Run-TryBot: Alex Brainman <alex.brainman@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/os/exec_windows.go

@@ -35,6 +35,12 @@ func (p *Process) wait() (ps *ProcessState, err error) {
 		return nil, NewSyscallError("GetProcessTimes", e)
 	}
 	p.setDone()
+	// NOTE(brainman): It seems that sometimes process is not dead
+	// when WaitForSingleObject returns. But we do not know any
+	// other way to wait for it. Sleeping for a while seems to do
+	// the trick sometimes.
+	// See https://golang.org/issue/25965 for details.
+	defer time.Sleep(5 * time.Millisecond)
 	defer p.Release()
 	return &ProcessState{p.Pid, syscall.WaitStatus{ExitCode: ec}, &u}, nil
 }