Currently some tests run in buffer mode and some don't. Those that don't can
corrupt stdout/stderr. Switch to using buffer mode everywhere so we're consistent.
If there is useful output on stdout/stderr, it will be displayed if the test
fails.
(From OE-Core rev: 978548c0abde2cb94c2782538552f39bdf2bf630)
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>