Bug 31795: (follow-up) Mock call to FindDuplicateAuthority

Bug 31795: (follow-up) Mock call to FindDuplicateAuthority

When running against Zebra, this tests fail because it requires the
record to be indexed timely. In theory, a 5 seconds delay should be
enough for the indexer to pick up. But that's not the point of the test,
and we should better just mock the query in this case.

To test:
1. Have ktd launch with zebra:
   $ ktd up -d
2. Run:
   $ ktd --shell
  k$ prove t/db_dependent/api/v1/authorities.t
=> FAIL: Tests fail!
3. Apply this patch
4. Repeat 2
=> SUCCESS: Tests pass!
5. Launch ktd with ES:
   $ ktd down
   $ ktd --es7 up -d
6. Repeat 2
=> SUCCESS: Tests pass!
7. Sign off :-D

Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>
Signed-off-by: David Nind <david@davidnind.com>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>