Bug 20996: (follow-up) Fix test

Bug 20996: (follow-up) Fix test

- Fixes the t/db_dependent/Illrequest/Config.t
- Adds tests for Koha::Illrequest::Config->has_branch

0) Do not apply the path
1) Run test t/db_dependent/Illrequest/Config.t
2) It fails - it is because the Koha/Illrequest/Config.pm prefixes
handling changed
3) Apply the patch
4) Run test again, it should pass now
5) Read through the test code and confirm it makes sense

Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>

Signed-off-by: Nick Clemens <nick@bywatersolutions.com>