MDL-35109 Improve unittests for cron based fetching of available updates

MDL-35109 Improve unittests for cron based fetching of available updates

The expected behaviour is as follows:

* If the recently fetched data is older than 48 hours, it is considered
  as outdated and the new fetch is executed
* else, if the recently fetched data is younger than 24 hours, it is
  considered as fresh enough and no fetch is executed
* else, if the current time is after 01:00 AM plus a certain offset
  (which is randomly generated for each site), the fetch is
  executed.