Bug 11247: Add a simple unit test for TransformHtmlToXml

Bug 11247: Add a simple unit test for TransformHtmlToXml

The unit test is db dependent (as Jonathan correctly remarked),
since the sub checks a preference.

Test plan:
Run t/db_dependent/Biblio/TransformHtmlToXml.t

Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>
(cherry picked from commit d1335a766d7b54c470701b0055fc99be2e7e7628)
Signed-off-by: Chris Cormack <chris@bigballofwax.co.nz>