Code Modernization: Remove dynamic properties in `Tests_*_Slashes`.

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.
Built from https://develop.svn.wordpress.org/trunk@53557


git-svn-id: http://core.svn.wordpress.org/trunk@53146 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-includes/version.php

@@ -16,7 +16,7 @@
  *
  * @global string $wp_version
  */
-$wp_version = '6.1-alpha-53556';
+$wp_version = '6.1-alpha-53557';
 
 /**
  * Holds the WordPress DB revision, increments when changes are made to the WordPress DB schema.