Bug 31557: (QA follow-up) Make tests more robust

Bug 31557: (QA follow-up) Make tests more robust

Tests had a few problems:

- They weren't wrapped in a transaction explicitly
- They called Koha::Biblios->delete!
- They failed if run a couple times

This patch:

- Wraps things in a transaction
- Removes not-required things ($target_rs variable not used)
- Preserves the tests logic, but filters the resultset on the
  biblionumber instead of deleting all the database which can fail
  depending on FK constraints.

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