Code Modernization: Rename the readonly() function to wp_readonly().

Since PHP 8.1, `readonly` is a reserved keyword and cannot be used as a function name.

In order to avoid PHP parser errors, the `readonly()` function was extracted to a separate file and is now only included conditionally on PHP < 8.1.

This commit also:
* Moves the tests for the `__checked_selected_helper()` function and all the related functions to their own file.
* Switches to named data providers. This makes the output when using the `--testdox` option more descriptive and is helpful when trying to debug which data set from a data provider failed the test.
* Improves the tests in question to make them feature-complete and expand test coverage.

Props jrf, ayeshrajans, haosun, knutsp, swissspidy, SergeyBiryukov.
Fixes #53858.
Built from https://develop.svn.wordpress.org/trunk@51586


git-svn-id: http://core.svn.wordpress.org/trunk@51197 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-includes/version.php

@@ -13,7 +13,7 @@
  *
  * @global string $wp_version
  */
-$wp_version = '5.9-alpha-51585';
+$wp_version = '5.9-alpha-51586';
 
 /**
  * Holds the WordPress DB revision, increments when changes are made to the WordPress DB schema.