Add and refactor unit tests for avatar functions

[pbearne]

Created new PHPUnit test classes for get_avatar, get_avatar_data, and get_avatar_url functions. These changes improve test organization and maintainability by grouping related tests together. Also, renamed avatar.php to Functions_GetAvatarUrl_Test.php to better reflect its content.

Trac ticket: https://core.trac.wordpress.org/ticket/53010

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