Bug 25623: Make sure oauth.t rolls back the db

Bug 25623: Make sure oauth.t rolls back the db

This patch adds a missing transaction wrapping one of the subtests. This
is probably an oversight when refactoring this file.

To test:
1. Open the patron categories page
2. Run:
   $ kshell
  k$ prove t/db_dependent/api/v1/oauth.t
=> SUCCESS: Tests pass
3. Reload the patron categories page
=> FAIL: There's random data in there
4. Apply this patch and repeat 2
5. Reload the patron categories page
=> SUCCESS: The random data from the previous tests is still there, but
no new random data
=> SUCCESS: Tests still pass
6. Sign off :-D

Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
Signed-off-by: Julian Maurice <julian.maurice@biblibre.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>