Bug 18826: rollback transaction for api tests

Bug 18826: rollback transaction for api tests

The holds.t tests for the REST api do no rollback properly and modify
the DB (no cleanup).
This comes from a bug caused by SessionStorage = mysql (default)

The error is:
    "rollback ineffective with AutoCommit enabled"

Test plan:
  select count(*) from borrowers;
  prove t/db_dependent/api/v1/holds.t
  select count(*) from borrowers;
=> The number of entry must be the same before and after the tests have
been executed

Signed-off-by: Josef Moravec <josef.moravec@gmail.com>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>