Support repository purging in prepare_test_env
    
    This introduces a "-n" argument which, if passed, will trigger a
    deletion of all repositories in the container. This is useful for
    speeding up tests which do not install additional packages and hence
    do not benefit from being slowed down for repository refresh operations.
    
    Signed-off-by: Georg Pfuetzenreuter <mail@georg-pfuetzenreuter.net>