Bug 34918: Fix userenv for safe_to_delete tests

Bug 34918: Fix userenv for safe_to_delete tests

Instead of the fallback to 51, we pass a borrower that has
permission for editing items.

Test plan:
Do not apply patch.
Run test without a borrower 51 in db. (FAIL)
Apply patch.
Run test again. (PASS)

Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Katrin Fischer <katrin.fischer@bsz-bw.de>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>
(cherry picked from commit f8233f44a5e4871be38d9b850d97166dcf99c981)
Signed-off-by: Fridolin Somers <fridolin.somers@biblibre.com>