oeqa: Default to buffer mode for tests

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.

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
Signed-off-by: Armin Kuster <akuster808@gmail.com>