Bug 18458: Add a subtest in Merge.t

Bug 18458: Add a subtest in Merge.t

This test illustrates the problem we have if you run it without the
second patch. And it serves to demonstrate that we resolved the
situation if you run it after the second patch.

Test plan:
[1] Without the second patch: The last subtest should fail.

Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Josef Moravec <josef.moravec@gmail.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>