If _run_command is called with block=True(default) then if the stderr or stdout > then the os buffer a deadlock could occur.

https://chromium.googlesource.com/chromiumos/third_party/autotest/+/master/client/cros/faft/utils/shell_wrapper.py#33

See the warning at https://docs.python.org/2/library/subprocess.html#subprocess.Popen.wait

We should be using .communicate()