Trim the test runner log to show only pytest failures/errors
    
    If we get to the `run-tests` phase, we no longer care about messages
    from previous steps so we can truncate the log output to only show
    pytest failures/errors, reducing log size.
    
    As a fallback, the previous behaviour to print last 5000 log lines was
    kept in the case the CI tests fail during setup.
    
    Reviewed-By: Stanislav Laznicka <slaznick@redhat.com>
    Reviewed-By: Fraser Tweedale <ftweedal@redhat.com>