ptest-runner: Pull in "runner: Remove threads and mutexes" fix

This fix simplifies the code to drop the pthread and mutexs which should
address some of the buffering issues we were seeing in some test case
failures.

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
(cherry picked from commit bffa6d38)
Signed-off-by: Steve Sakoman <steve@sakoman.com>