Generate test files on the fly (first version)

Note that this version is missing code to do this for *corrupt*
archives, i.e. those that result in an IOError ("IOError: [Errno 22]
Invalid argument"; zipfile.py, line 781, call to fp.seek()) when using
them to instantiate a zipfile.ZipFile object.
Make sure corrupt archive is deleted from upload dir after testing zip
integrity validator
Refactor validator tests