oeqa/ptest: print a warning if ptests failed

This allows spotting ptest regressions without having hard ptest failures
(for that full ptest stability should be achieved).

Signed-off-by: Alexander Kanavin <alex.kanavin@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
(cherry picked from commit 79b54d90)
Signed-off-by: Steve Sakoman <steve@sakoman.com>
(cherry picked from commit 5232c0b480eba4537c8db71c95189950011fb64d)
Signed-off-by: Steve Sakoman <steve@sakoman.com>