Bug 9988: Add a subtest for specific postponed merge issues

Bug 9988: Add a subtest for specific postponed merge issues

This subtest shows that we need a few little tweaks to make merge
work in some specific postponed merge situations.

Test plan:
[1] Run t/db_dependent/Authorities/Merge.t.
    The last subtest should fail.

Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Marc Véron <veron@veron.ch>
Signed-off-by: Jacek Ablewicz <abl@biblos.pk.edu.pl>
Signed-off-by: Julian Maurice <julian.maurice@biblibre.com>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>