Add tests for Realtime class
Add tests for summer and winter timezone handling
Implement fix for timezone

As for as we can tell, the process for generating realtime timestamps
used to go:

- generate timestamp now for utc e.g. 10:20 am +00:00
- change timezone to gmt without affecting the rest of the time e.g. 10:20 am +01:00
- send to backdrop
- convert back to utc without affecting the rest of the time e.g. 10:20 am +00:00
- save, timestamps as they are now will be sent to spotlight.

This puts the time coming to spotlight one hour back vs local time

The fix:

- generate timestamp now for gmt e.g. 11:20 am +01:00 when BST
- send to backdrop
- convert back to utc without affecting the rest of the time e.g. 11:20 am +00:00
- save, timestamps as they are now will be sent to spotlight.

Tests force the system time to a summer time and winter one and check we
are sending the correct timestamp to backdrop.