Re-architect wpdb::init_charset() for unit testing

[pento]

The behaviour of wpdb::init_charset() is becoming more complex, but due to the dependence on defines for passing data, instead of parameters, it's not possible to test that behaviour.

By splitting up the logic, we can add testing for this method.

See [37522].

[pento]

First pass.

[pento]

In 37601:

Database: Split the logic of wpdb::init_charset() into a separate method.

The logic for determining the appropriate character set and collation to use is becoming more complex, particularly with the recent additions of [37522] and [37523]. As init_charset() has side effects, and makes use of constants instead of parameters, it's not possible to unit test this logic.

This commit splits the logic part of init_charset() out into a new method, wpdb::determine_charset(), along with appropriate unit tests.

See #32105, #37522.

Fixes #36917.

[pento]

In 37602:

Tests: Fix an incorrect @ticket header introduced in [37601].

See #32405, #36917.