Bug 17909: Add unit tests for C4::AuthoritiesMarc::merge

Bug 17909: Add unit tests for C4::AuthoritiesMarc::merge

Original patch from Julian Maurice on bug 11700.
With sign offs by:
Signed-off-by: Bernardo Gonzalez Kriegel <bgkriegel@gmail.com>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>
Amended by Marcel de Rooy on report 17909.

EDIT (January 2017):
Removed some tests not related to merge.
Put remaining tests in a subtest, made them working on current merge.
Slightly revised the mocking.

Note: I plan to move the zebra retrieval stuff outside merge in one of
the next stages, and replace it by calling Koha::SearchEngine. This will
reduce mocking complexity here.

Test plan:
Just run t/db_dependent/Authorities/Merge.t

Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Josef Moravec <josef.moravec@gmail.com>
Signed-off-by: Julian Maurice <julian.maurice@biblibre.com>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>