Bug 8252: (follow-up) make it easier to add UNIMARC indexing tests

Bug 8252: (follow-up) make it easier to add UNIMARC indexing tests

This patch refactors t/db_dependent/Search.t so that it can be
subsequently modified to add tests for indexing and searching
UNIMARC records without duplicating the setup code.

This patch also moves the MARC21 records from t/db_dependent/data/zebraexport
to t/db_dependent/data/marc21/zebraexport so that UNIMARC sample
data can be slotted in t/db_dependent/data/unimarc/zebraexport.

To test:

[1] Verify that prove -v t/db_dependent/Search.t passes.

Signed-off-by: Galen Charlton <gmc@esilibrary.com>