Dynamic (non-explicitly declared) properties are deprecated as of PHP 8.2 and are expected to become a fatal error in PHP 9.0.
In this particular case, the test classes contain a `set_up()` method that sets a group of properties, which are ''used'' by the tests, but the values of these properties are never ''changed'' by the tests.
In other words, setting these properties in the `set_up()` is an unnecessary overhead and the properties should be changed to class constants.
Follow-up to [1041/tests], [1071/tests].
Props jrf.
See #56033.
git-svn-id: https://develop.svn.wordpress.org/trunk@53557 602fd350-edb4-49c9-b593-d223f7449a82