Bug 15432: t/db_dependent/Letters.t depends on external data/configuration

Bug 15432: t/db_dependent/Letters.t depends on external data/configuration

To test:

- Set system preference LetterLog to 'Don't log'
- prove t/db_dependent/Letters.t
- Result: Fails on test 62 - Successfully sent claim
- Apply patch
- Test again
- Result: Pass OK

Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
Amended patch: removed the bug reference, not needed.

Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>
(cherry picked from commit c92b1556ef9250d7e038dce2d083c60415035b95)
Signed-off-by: Julian Maurice <julian.maurice@biblibre.com>