Bug 31920: Add transaction to subtest in t/db_dependent/Holds.t

Bug 31920: Add transaction to subtest in t/db_dependent/Holds.t

A subest in t/db_dependent/Holds.t leaves behind database cruft.

Test Plan:
Using koha-testing-docker:
1) SELECT branchcode FROM branches
2) prove t/db_dependent/Holds.t
3) SELECT branchcode FROM branches
   Note cruft
4) reset_all
2) prove t/db_dependent/Holds.t
3) SELECT branchcode FROM branches
   No cruft!

Signed-off-by: David Nind <david@davidnind.com>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>