Bug 21360: Prevent IssueSlip.t to fail when run at 23:59

Bug 21360: Prevent IssueSlip.t to fail when run at 23:59

Not sure what happened on bug 15029 (Make the IssueSlip tests pass even if
launched after 23:00) but we should have tested with 23:59, not 23:50...

koha_1       |     # Looks like you planned 3 tests but ran 2.
koha_1       |
koha_1       | #   Failed test 'Issue slip'
koha_1       | #   at t/db_dependent/Members/IssueSlip.t line 294.
koha_1       | Label not found for "last SKIP" at /usr/share/perl5/Test/More.pm line 1370.
koha_1       | # Looks like your test exited with 255 just after 1.
koha_1       | [23:59:10] t/db_dependent/Members/IssueSlip.t
koha_1       | Dubious, test returned 255 (wstat 65280, 0xff00)
koha_1       | Failed 3/3 subtests

Test plan:
0/ Don't apply this patch
1/ sudo date -s "2018-09-15 23:59:00";prove t/db_dependent/Members/IssueSlip.t
=> You should get the error
2/ Apply this patch and repeat 1.
=> Tests should pass now!

Signed-off-by: Nick Clemens <nick@bywatersolutions.com>