Bug 18356: Extend GetNextDate.t, add GetFictiveIssueNumber.t (unit=year)

Bug 18356: Extend GetNextDate.t, add GetFictiveIssueNumber.t (unit=year)

This patch deals with tests for yearly frequencies.

Adjust/extend GetNextDate.t:
[1] Adjust mixup of units/issues in a description.
[2] Add testing +2 years on 29-2 of leap year for freq 1 issue/2 years.
[3] Add tests for freq 9 issues/year.

Add GetFictiveIssueNumber.t:
[1] Two subtests are provided for irregular frequencies (very trivial) and
    for year frequencies (with four specific test cases).

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

Note: Without the second patch both tests should fail. This shows the need
of the adjustments in the second patch.

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>