tests: attempting to avoid intermittent EINTR failures

I suspect we are hitting this Python bug:

http://bugs.python.org/issue12493

We can simply capture and discard the commands' stderr, it is not
expected to be large and otherwise it would just be lost anyway (since
stderr is redirected to /dev/null).

Change-Id: I1c6fcc6fac66db2368bd00623ee8f8ff147b3a2f
Merge "tests: attempting to avoid intermittent EINTR failures" into develop