Set current user to 0 after each unit test.

This prevents $current_user, $userdata, and other user-related globals from
being polluted if a previous test does not properly reset the current user.

Props nacin, mnelson4.
Fixes #29712.
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Editor: use <button> instead of <a> for the Visual/Text buttons, make them focusable. Props afercia, fixes #27553
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Add unit tests for WP_Comment_Query 'orderby' param.

For better testability, the SQL string generated in `WP_Comment_Query::get_posts()`
is now stored as a 'request' property on the object.

See #29902.
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Allow ORDER BY in `WP_Comment_Query::query()` to be disabled.

Disable ORDER BY by passing boolean false, an empty array, or the string
'none' to the 'orderby parameter. This mirrors the behavior of `WP_Query`.

Props psycleuk.
Fixes #29902.
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