Bug 18697: Adjusting unit tests for weekly serial frequencies

Bug 18697: Adjusting unit tests for weekly serial frequencies

Corrections and added unit tests following the changes of the first patch.

GetFictiveIssueNumber.t: New subtest for weekly frequencies.

GetNextDate.t: Correcting a few dates one day. If we use 2/week, we will
calculate an interval of 3 days and correct with 4 days at the end of
the cycle. The connection with firstacqui is not relevant anymore.

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>