Bug 17501: Additional polishing (POD, unit tests)

Bug 17501: Additional polishing (POD, unit tests)

This patch adds some documentation lines.
And mainly rearrangs the tests in Upload.t. The 'basic CRUD testing' is
not needed separately any more. A new test catches the "file missing"
warn.

Test plan:
[1] Run perldoc on UploadedFile[s].pm
[2] Run t/db_dependent/Upload.t

Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>