oeqa/runner: Ensure class setup errors are shown to bitbake logging

oeqa/runner: Ensure class setup errors are shown to bitbake logging

This took a bit of digging but failure messages from testimage are shown to bitbake's
logging through stopTest. In the case of a setUpClass failure stopTest is never
called and the bitbake logging never sees the error. It would still be in the task
logfile. Add some code+comment to ensure logs not shown to the user mid stream are shown
at the end.

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>