The E2E test for periodic dispatch jobs has a `cron` trigger for once a
minute. If the test happens to run at the top of the minute, it's possible for
the forced dispatch to run from the test code, then the periodic timer triggers
and leaves a running child job. This fails the test because it expects only a
single job in the "dead" state.
Make it so that the `cron` expression is implausible to run during our test
window, and migrate the test off the old framework while we're at it.
