Summary

Unit tests currently fail in trunk, based on the 6.3 release from yesterday. There are two problems that lead to those failures:

• WordPress 6.3 makes the Fetchpriority module useless. The module is already not being loaded with that version, so we need to ensure we don't run tests in that case either.
• WordPress 6.3 actually comes with a breaking change in a low-level function which we use in the WebP Uploads module: Per https://core.trac.wordpress.org/ticket/57685 / https://core.trac.wordpress.org/changeset/55935, editing thumbnails (or all image sizes other than thumbnails) is no longer enabled by default, which is technically a breaking change 😞

This PR brings parity with those changes:

• Handle the image editing target values thumbnail and nothumb only when editing thumbnails separately is enabled (if using WP 6.3 or greater, per the new filter).
• Skip tests that aren't relevant based on the related new WP 6.3 functionality.

Checklist

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