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.

git-svn-id: https://develop.svn.wordpress.org/trunk@51586 602fd350-edb4-49c9-b593-d223f7449a82

phpunit.xml.dist

@@ -5,6 +5,7 @@
 		backupGlobals="false"
 		colors="true"
 		beStrictAboutTestsThatDoNotTestAnything="true"
+		beStrictAboutOutputDuringTests="true"
 		>
 	<testsuites>
 		<!-- Default test suite to run all tests. -->