test-def: create SLO definitions in setup so SLO read ops have data

Opened from failing KB functional tests in `test/functional/kb/definitions/`. SLO read ops fail with `404 Not Found` on the stack run because no SLO definitions exist, so `get`/`snapshot`/`bulk_snapshot` have no data to operate on.

Create SLO definitions in setup (or provide a fixture) and capture their ids, then reference them in the affected definitions so the read ops have data. Add teardown to remove the created SLOs.

Failing test definitions (stack):
- `slo_bulk_snapshot_op.yml`
- `slo_get_definitions_op.yml`
- `slo_get_snapshot_op.yml`