Add formula cloning to prepare_test_env
    
    Clone Git based formulas as part of prepare_test_env.
    This avoids us having to repeatedly call clone_formulas.sh in the
    CI configuration, since it is usually always needed together
    with prepare_test_env.sh.
    In cases where it is not desired, the new feature can be skipped
    using "-c".
    
    Signed-off-by: Georg Pfuetzenreuter <mail@georg-pfuetzenreuter.net>