bufio.NewReader.ReadString blocks even
when the process received syscall.SIGKILL.
Remove ptyMu mutex and make ReadString return
when *os.File is closed.
Signed-off-by: Gyuho Lee <gyuhox@gmail.com>
add and expose StopSignal to ExpectProcess allows user
to define what signal to send on ExpectProcess.close()
coverage testing code sets StopSignal to SIGTERM allowing
the test binary to shutdown gracefully so that it can generate
a coverage report.
