Bug 14494: Add some unit tests too for CalcDateDue

Bug 14494: Add some unit tests too for CalcDateDue

The second patch of this report made some changes to CalcDateDue.
We are adding some unit tests here.
See the commments on the third patch too.

Test plan:
Run t/db_dependent/Circulation_dateexpiry.t

Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
Amended patch: Set the number of tests for the second subtest (was
commented) and perltidy the second block.

Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>
Signed-off-by: Tomas Cohen Arazi <tomascohen@unc.edu.ar>
(cherry picked from commit 1a8962f306d2d2f0c7789c649aa615192ccd5ca1)
Signed-off-by: Chris Cormack <chrisc@catalyst.net.nz>