ptest-runner: Pull in sync fix to improve log warnings

Pulls in:

    utils: Ensure buffers are flushed after child exits

    We currently wait for the child to exit but we don't flush the buffers.
    This can mean the output ends up out of sync and the END: line isn't at
    the end of the logs.

    We've recently seen a lot of issues related to this on the autobuilder.
    Add in a flush call for all fds to ensure buffers are in sync. This
    does appear to improve warnings on the autobuilder now we started detecting
    the issue.

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
(cherry picked from commit f051de5d)
Signed-off-by: Steve Sakoman <steve@sakoman.com>