Bug 36493: Make cashups test more robust

Bug 36493: Make cashups test more robust

This patch makes the test independent from existing cash registers in
the system.

To test:
1. Enable UseCashRegisters on a fresh KTD
2. Add a cash register
3. Run:
   $ ktd --shell
  k$ prove t/db_dependent/api/v1/cashups.t
=> FAIL: Tests fail!
4. Apply this patch
5. Repeat 3
=> SUCCESS: Tests pass!
6. Sign off :-D

Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>
Signed-off-by: David Nind <david@davidnind.com>
Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Katrin Fischer <katrin.fischer@bsz-bw.de>