PHPunit tests for _wp_timezone_choice_usort_callback

[jorbin]

In 57145:

Add automated tests for _wp_timezone_choice_usort_callback

Props pbearne.
Fixes #59953.

[SergeyBiryukov]

In 57146:

Tests: Update _wp_timezone_choice_usort_callback() tests for consistency.

• Use the same @group annotation as the other tests.
• Use assertSame() to verify the type of the result.
• Use data_ prefix for the data provider.
• Use named data set in the data provider. This makes the output when using the --testdox option more descriptive and is helpful when trying to debug which data set from a data provider failed the test.
• Other minor corrections.

Reference: ​Core Handbook: Writing PHP Tests: Repetitive Tests.

Follow-up to [57145].

See #59953, #59647.

Thanks for the PR! This was merged in r57145.