oeqa/selftest/bbtests: add non-existent prefile/postfile tests

Fixes [YOCTO #10725]

Signed-off-by: Fabien Mahot <fabien.mahot@smile.fr>
Reviewed-by: Yoann Congal <yoann.congal@smile.fr>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
(cherry picked from commit b0c33655)
Signed-off-by: Steve Sakoman <steve@sakoman.com>