Update to WordPress 5.9.2

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WordPress 5.0 was released on 6th Dec 2018.
The code should be updated to reflect the latest level.

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There have been quite a few releases since WordPress 5.0. And WordPress 5.3 is now on its way.

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We also need to update to a supported version of PHPUnit. The latest currently available is phpunit-8.4.1.phar.

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Latest version of WordPress is 5.9.2. It would be a good idea to update now if I'm going to attempt to develop some failing tests for a permalink issue.