Bug 34825: Merge Letters.t into t/db/Letters.t

Bug 34825: Merge Letters.t into t/db/Letters.t

Test plan:
Run t/db_dependent/Letters.t

Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>
(cherry picked from commit 5fd831755205fc3b3b7481103fbe2145b421a5ea)
Signed-off-by: Fridolin Somers <fridolin.somers@biblibre.com>
(cherry picked from commit f938f90795d5c57e59d4fc38de5b8fd0991f354f)
Signed-off-by: Matt Blenkinsop <matt.blenkinsop@ptfs-europe.com>