Bug 30838: (QA follow-up) Add unit tests

Bug 30838: (QA follow-up) Add unit tests

Two tests added, one in t/db_dependent/Circulation.t to catch the
initial setting of to_address at enqueue time and a second in
t/db_dependent/Letters.t to catch the correcting at send time.

Signed-off-by: Katrin Fischer <katrin.fischer@bsz-bw.de>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>
(cherry picked from commit 467b444c7e355b1ef3ecbab29f67544bfd72360b)

Signed-off-by: Lucas Gass <lucas@bywatersolutions.com>
(cherry picked from commit 899898400f095a0cfbbae950bf2d46b8327db905)
Signed-off-by: Arthur Suzuki <arthur.suzuki@biblibre.com>