Bug 37623: Make t/db_dependent/Letters.t idempotent

Bug 37623: Make t/db_dependent/Letters.t idempotent

The tests should be unaffected by system preference settings, such as EmailFieldPrimary.

To test:
1. Log into the staff interface
2. Go to Koha Administration -> Global system preferences, and search for EmailFieldPrimary
3. Change the EmailFieldPrimary preference to something other than 'primary email' and 'first valid email address', i.e. any of the following:

* alternate email
* card number
* secondary email
* selected addresses

4. Run tests t/db_dependent/Letters.t and notice they fail

5. Apply the patch

6. Run the tests again t/db_dependent/Letters.t with different settings of EmailFieldPrimary. The tests should always pass.

Sponsored-by: Pymble Ladies' College
Signed-off-by: Paul Derscheid <paul.derscheid@lmscloud.de>
Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Katrin Fischer <katrin.fischer@bsz-bw.de>