Bug 28565: Refactor tests

Bug 28565: Refactor tests

Tests were too complex with no gain. They even deleted the whole DB to
run (inside a transaction). I refactored to test the same thing, and
make it rely (mostly) on existing data.

_count behavior is tested implicitly by comparing with Koha::*->count.

To test:
1. Run:
   $ kshell
  k$ prove t/db_dependent/UsageStats.t
=> SUCCESS: Tests pass
2. Apply this patch
3. Repeat 1
=> SUCCESS: Tests still pass
4. Sign off :-D

Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>
Signed-off-by: David Nind <david@davidnind.com>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>