concurrencytest: Fix Python3 warning

This gives a warning in some situations:

  File "tools/dtoc/../concurrencytest/concurrencytest.py", line 95,
       in do_fork
    stream = os.fdopen(c2pread, 'rb', 1)
  File "/usr/lib/python3.8/os.py", line 1023, in fdopen
    return io.open(fd, *args, **kwargs)
RuntimeWarning: line buffering (buffering=1) isn't supported in binary
    mode, the default buffer size will be used

Fix this by dropping the line-buffer parameter.

Signed-off-by: Simon Glass <sjg@chromium.org>