Bug 25638: Add tests for JSON conversion vs DBD::mysql

Bug 25638: Add tests for JSON conversion vs DBD::mysql

We have the situation of needing to cast numbers so they are not
confused in JSON with strings by DBD::mysql returning them with the
wrong internal flags.

We have also recently removed the cast, and adding it back now.
This test could help us detect this situation in case we make changes in
the area, and even on specific libraries versions.

Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>

Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>