Bug 33360: Adjust Letters.t for limit parameter and domain limits

Bug 33360: Adjust Letters.t for limit parameter and domain limits

Includes:
[1] Add a counter in the send_or_die mock.
[2] Correct from processed count to sent count.
[3] More extensive testing for limit parameter and domain limits.

Test plan:
Run t/db_dependent/Letters.t

Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>