Bug 18601: OAI/Sets.t mangles data due to truncate in ModOAISetsBiblios

Bug 18601: OAI/Sets.t mangles data due to truncate in ModOAISetsBiblios

This patch replaces the TRUNCATE statement in ModOAISetsBiblios by a
DELETE statement. A truncate will cause an implicit commit and will
therefore commit the transaction started in the test script.

Also simplifying the module load in the test script.

Test plan:
Do not apply this patch and observe that biblio records are added to your
database by running t/db_dependent/OAI/Sets.t.
Apply this patch, run the test again and verify that it does no longer
add records to your biblio table.

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>