oeqa/sysroot.py: Check bitbake return status

bitbake ran but we incorrectly did not assert the exit status needs to
be non 0. Now all sysroot tests commands expected to fail are verified
to do so.

Signed-off-by: Paulo Neves <ptsneves@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>