Maildigest tests can query a wrong adhoc task

Mail digest tests collect all records from the task_adhoc table. Other plugins (enrol_auto, enrol_coursecompleted, format_tiles, ...) also add records to this table, so the queries in this test have to be limited to this component only.

Replication steps:

1. Install a plugin that creates an adhoc task upon an event (enrol_auto, enrol_coursecompleted, format_tiles, ...)
2. Run php unit tests

Expected: No errors
Actual: 2 errors assertGreaterThanOrEqual(time(), $task->nextruntime)