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.
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