Bug 14693: Contract.t should generate dates based on current date

Bug 14693: Contract.t should generate dates based on current date

The current test for still-active contracts fails because it was written
with fixed dates, which already passed :-D

This patch calculates the current date, and uses it to generate a future
one, so the tests serve their purpose no matter what the current date is.

To test:
- Run:
  $ prove t/db_dependent/Contract.t
=> FAIL: activeonly compares contractenddate with now(), and fails
         because a fixed 2015-07-31 is writteng on the test data
- Apply the patch
- Run:
  $ prove t/db_dependent/Contract.t
=> SUCCESS: the sample data has been calculated using the current date
            and the tests pass.
- Sign off :-D

Edit: added missing use DateTime::Duration,

Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
Signed-off-by: Tomas Cohen Arazi <tomascohen@unc.edu.ar>