Bug 22254: Correction to datehandling in test

Bug 22254: Correction to datehandling in test

Adding 13 months is not always the same as adding 1 month and then
adding 12 months in DateTime maths.  This patch rectified the test
to match the correct behaviour in koha.

Test plan.

1) Modify the test (around line 268) to test against the suspect
   leap date of 2019-01-31 and watch the test fail.
2) Apply the patch (which also adds this modification to test
   for such a leap date) and watch the test now pass.

Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>