Bug 27317: (Bug 27127 follow-up) fix t/db_dependent/Koha/BackgroundJobs.t

Bug 27317: (Bug 27127 follow-up) fix t/db_dependent/Koha/BackgroundJobs.t

Bug 27127 changed  Koha::BackgroundJob->messages to return arrayref instead of array.
This impacts Unit Test t/db_dependent/Koha/BackgroundJobs.t failling on :
  not ok 10 - 10 messages generated

Test plan :
prove t/db_dependent/Koha/BackgroundJobs.t

Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>
Edit: I fixed the indentation of the test label.
Signed-off-by: Victor Grousset/tuxayo <victor@tuxayo.net>
(cherry picked from commit 5f2414686dbba56ee8140cad55051d7b900cb47d)
Signed-off-by: Fridolin Somers <fridolin.somers@biblibre.com>