[php-src] Issue #22961: `run-tests.php` should be split up

Issue: https://github.com/php/php-src/issues/22961
Author: NickSdot

### Description

`run-tests.php` is so big that it is really hard to navigate, and GitHub does not show diffs for it by default. Would we be keen to split it up into multiple sources and generate them back into a single executable at its existing location on `make`? If so, I would take a stab at it after the #22917-related work is done.