tests: Add tests for timezone validity

For every location at the country level, or smaller, check whether we
have at least one timezone defined. If only one timezone is defined for
the location, verify its validity.

This generated 3 errors:
(MSG: Location 'Anywhere on Earth (AoE)' has invalid timezone 'Etc/UTC-12')
(MSG: Location 'Wake Island, Wake Island Army Airfield Airport' does not have an associated timezone)
(MSG: Location 'Wake Island, Wake Island Army Airfield Airport' does not have an associated timezone)

https://bugzilla.gnome.org/show_bug.cgi?id=791307