[meta] Define a Rector rule to convert test annotations to attributes

Currently developing a Rector script for this in https://github.com/mondrake/d8-unit/blob/test-rector/rector.php

Updated IS to clarify this is required in PHPUnit 12, not 11.

The Needs Review Queue Bot → tested this issue. It fails the Drupal core commit checks. Therefore, this issue status is now "Needs work".

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'Active' is more appropriate as a status, maybe.