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>