Bug 17726: TestBuilder - Add default values

Bug 17726: TestBuilder - Add default values

The items.more_subfields_xml is set to random data (generated by
TestBuilder), and so GetMarcBiblio does not manage to embed items (if
needed).

The error is:
  :1: parser error : Start tag expected, '<' not found

More precisely it explodes in
C4::Items::_parse_unlinked_item_subfields_from_xml when
MARC::Record->new_from_xml is called with an invalid xml

This patch adds a default values mechanism to TestBuilder to avoid
modifying all the existing calls.

Test plan:
Set SearchEngine to ElasticSearch
prove t/db_dependent/Circulation.pl
should return green with this patch

Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>