Bug 18607: Adjusting unit tests for monthly serial frequencies

Bug 18607: Adjusting unit tests for monthly serial frequencies

The changes in the first patch require some corrections as well as
additional test cases.

GetNextDate.t: Since the calculation for multiple issues per unit has
slightly changed, a few dates (day 15 or day 16) have been changed in the
unit test; when we use 2/month, the algorithm now always adds 15 days.
Added a few test descriptions in this regard too.

GetFictiveIssueNumber.t: Add the monthly subtest with two test cases. In
the first case we tests multiple units per issue, and in the second case
we test multiple issues per unit (month).

Test plan:
[1] Run t/db_dependent/Serials/GetFictiveIssueNumber.t
[2] Run t/db_dependent/Serials/GetNextDate.t

Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Josef Moravec <josef.moravec@gmail.com>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
(cherry picked from commit 05a49035ccff1d962ef0c21ca1b154e07ba5dba7)
Signed-off-by: Fridolin Somers <fridolin.somers@biblibre.com>
(cherry picked from commit 45c31333a879e35ecf7ddc12d5305002c80ba065)
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>