Fix and improve testing uploading a file to a ticket
    
    This commit adds unit-tests for uploading multiple files at once in the
    UI as well as rework how the testing for single upload is done so that
    it is tested even when pyclamd is not installed/available.
    
    Signed-off-by: Pierre-Yves Chibon <pingou@pingoured.fr></pingou@pingoured.fr>