Bug 33870: Polishing Sitemapper.t

Bug 33870: Polishing Sitemapper.t

Create subtest.
Remove (useless) mock on DateTime.
Remove T::D::C module, add TestBuilder.
Replace local slurp function by use File::Slurp.

Test plan:
Run t/db_dependent/Sitemapper.t

Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Pedro Amorim <pedro.amorim@ptfs-europe.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>