Split the tests for `wp_json_encode()` into smaller chunks (let's all them "units"). Skip a couple of these tests when running on older versions of PHP that don't support the tested functionality.

See #28786.

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git-svn-id: http://core.svn.wordpress.org/trunk@30522 1a063a9b-81f0-0310-95a4-ce76da25c4cd
`json_encode()` returns different results for non UTF-8 strings in PHP 5.5+, versus earlier versions of PHP.

This fixes the unit tests that fail in earlier versions, see #30471 for fixing this globally in `wp_json_encode()`.

Built from https://develop.svn.wordpress.org/trunk@30534


git-svn-id: http://core.svn.wordpress.org/trunk@30523 1a063a9b-81f0-0310-95a4-ce76da25c4cd