Bug 6874: Add unit tests for C4::UploadedFiles

Bug 6874: Add unit tests for C4::UploadedFiles

Add unit tests for C4::UploadedFiles and move a variable
declaration at subroutine level instead of file level.
Add dependency to Test::CGI::Multipart

Still works, and the newly-provided unit tests have good test
coverage:
C4/UploadedFiles.pm 90.7    65.0    66.7    100.0   100.0    0.2    86.4

Signed-off-by: Jared Camins-Esakov <jcamins@cpbibliography.com>
Signed-off-by: Mark Tompsett <mtompset@hotmail.com>
Signed-off-by: Bernardo Gonzalez Kriegel <bgkriegel@gmail.com>
Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Tomas Cohen Arazi <tomascohen@unc.edu.ar>