openssh/util-linux/python*: Ensure ptest output is unbuffered

openssh/util-linux/python*: Ensure ptest output is unbuffered

We need to run sed with the -u option to ensure the output is unbuffered else
ptest-runner may timeout thinkig things were idle. Busybox doesn't have the -u
option so we need to RDEPEND on sed (which is a good thing to do if we use it
anyway).

Alex Kanavin should get credit for discovering the problem.

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>