Add test coverage for default time zone in tests

Problem/Motivation

In most of the date/time/timezone testing in core, we assume that the PHP environment and default time zone has been set. We do this in the phpunit bootstrap and other test base setups, but we don't have explicit test coverage to catch someone changing this and getting fails in tests that are calibrated to 'Australia/Sydney'.

Proposed resolution

Add assertions that date_default_timezone_get() is 'Australia/Sydney' in the various test base tests w/ comment. This will make a inadvertant test failure easier to pinpoint.

Remaining tasks

User interface changes

API changes

Data model changes