Bug 26250: Fix tests when SearchEngine=Elastic

Bug 26250: Fix tests when SearchEngine=Elastic

Most of the time the tests are failing because the item is not created
correctly (missing biblio and/or biblioitem).
The usual error is:
 t/db_dependent/selenium/regressions.t ..... 5/5 Can't call method "leader" on an undefined value at /kohadevbox/koha/Koha/SearchEngine/Elasticsearch.pm line 534.

In this patch we are making sure $builder->build({ source => 'Item' })
is replace with $builder->build_sample_item

Test plan:
Turn on Elastic and confirm that all the tests pass!

Signed-off-by: Nick Clemens <nick@bywatersolutions.com>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>