python-avocado.spec: be a little more forgiven about the selftests errors
    
    We've spent quite some time trying to identify the ones that are
    sensitive to time and resources, so let's document what (usually)
    causes failures at package build time.
    
    Also, let's make it clearer that they are run by default.
    
    Signed-off-by: Cleber Rosa <crosa@redhat.com>