Bug 19042: Silence warnings t/db_dependent/Letters.t

Bug 19042: Silence warnings t/db_dependent/Letters.t

TEST PLAN
---------
prove t/db_dependent/Letters.t
-- there will be a message: "C4::Context->userenv not defined!"
apply this patch
prove t/db_dependent/Letters.t
-- there will no longer be that message.
run qa test tools

Signed-off-by: Lee Jamison <ldjamison@marywood.edu>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
(cherry picked from commit d01ce104a7711814152e4f8228064f79c8a7f68c)
Signed-off-by: Fridolin Somers <fridolin.somers@biblibre.com>