Add tests checking the behavior when uploading two files and fix two bugs
    
    The two bugs were:
    - Ensure we run update the checksums only once, even if we upload two
      files, otherwise, the first file uploaded ends up twice in the
      checksums file.
    - Ensure we do not write twice the header line when uploading two files
      to a project that did not have a checksums file before.
    
    Signed-off-by: Pierre-Yves Chibon <pingou@pingoured.fr>