Bug 14206: Adds test for getletter() call from overdue_notices.pl

Bug 14206: Adds test for getletter() call from overdue_notices.pl

Adds missing test for getletter() when called from overdue_notices.pl

Test plan
=========

1/ apply this patch
2/ run prove -v t/db_dependent/Letters.t
   all tests should pass, especially test #40 which tests call from
   overdue_notices.pl

Signed-off-by: Marc Véron <veron@veron.ch>
Signed-off-by: Jonathan Druart <jonathan.druart@koha-community.org>
Signed-off-by: Tomas Cohen Arazi <tomascohen@gmail.com>