As we are introducing a FK for illrequests.biblio_id, TestBuilder is
generating a linked biblio, and so tests expecting undef are failing.
This fixes it by explicitly setting biblio_id => undef on creating the
request.
The patch also cleans the area a bit, removing a redundant and fragile
test as well.
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>