Fix runtests.py so it works in containers
    
    Basically in a container we want to mount the results directory
    so the test results are exported outside of the container.
    This means, the results directory cannot be deleted or created
    from inside the container. So if we can delete it, let's just
    remove all the files it contains so we start fresh.
    
    Signed-off-by: Pierre-Yves Chibon <pingou@pingoured.fr>