Add a CRUD test (create/fetch/update/delete) for the Cronjob class,
which had no test coverage yet.
Scope: only the job definition CRUD is covered. run_jobs()/
reprogram_jobs() actually execute the configured command/method,
which is out of scope for a unit test - the job created here is left
disabled (STATUS_DISABLED) so nothing in the environment could pick
it up and run it.
The module is auto-activated in setUpBeforeClass() if not already
enabled, following the same pattern as the other recently added
tests (activation is real and persists after the test run, it is not
undone by the rollback in tearDownAfterClass - see comment in
setUpBeforeClass() for why).
