When using `clock::now()` the frozen cloxk and incrementing clock will now
always return a `DateTimeImmutable` that uses the configured Moodle system
time. In unit tests this will be Australia/Perth.
Before this change, it would sometimes be UCT.
This commit adds the PSR-20 ClockInterface to core, with a
moodle-specific extension to the Interface at `\core\clock`, and a
standard clock at `\core\system_clock`.
Further clocks are provided as `\incrementing_clock` and `\frozen_clock`
which are available to unit tests using:
- `$this->mock_clock_with_incrementing(?int $starttime = null);`
- `$this->mock_clock_with_frozen(?int $time = null);`
For the incrementing clock, every call to fetch the time will bump the
current time by one second.
For the frozen clock the time will not change, but can be modified with:
- `$clock->set_to(int $time);`; and
- `$clock->bump(int $seconds = 1);`
Neill Magill