Bug 17950: Improve quick UNIMARC fix in Merge.t

Bug 17950: Improve quick UNIMARC fix in Merge.t

Bug 17909 and 17913 added a quick fix for Merge.t on UNIMARC records.
This patch improves that fix with the sub compare_fields, a merge from
compare_field_count and compare_field_order.
Also it adds the option to test MARC21/UNIMARC by adding a command line
switch that triggers mocking the marcflavour preference.
The test on a cleared field 609 in strict mode has been broken up in two
tests: first a count without 609 and then counting 609s only.

Note: Could have mocked GetMarcBiblio too, but decided to go this way.

Test plan:
[1] Run perl t/db_dependent/Authorities/Merge.t
[2] (For UNIMARC users:) Run perl Merge.t -flavour MARC21
[3] (For others:) Run perl Merge.t -flavour UNIMARC

Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Mark Tompsett <mtompset@hotmail.com>
Signed-off-by: Julian Maurice <julian.maurice@biblibre.com>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>