[php-src] Issue #15631: Organize `Zend/tests/*.phpt` tests into subdirectories for easier navigation

Issue: https://github.com/php/php-src/issues/15631 Author: DanielEScherzer ### Description I wanted to take a look at any of the enum-related tests, but on GitHub navigating to https://github.com/php/php-src/tree/master/Zend/tests shows > Sorry, we had to truncate this directory to 1,000 files. 1,534 entries were omitted from the > list. Latest commit info may be omitted. I propose that the tests that are directly in the top-level Zend/tests/*.phpt be organized a bit with sub directories so that they can be navigated more easily, both on GitHub and in downloaded copies of PHP. For an easy example, there are 21 different tests for creating class aliases, class_alias_001.phpt through class_alias_021.phpt - these could be grouped together in their own directory (probably just named class_alias). I'm happy to send the patches to work on this reorganization if there is buy-in, but figured I should ask before I started the work. Since this doesn't affect the actual execution of PHP I didn't think it was applicable to propose on the internals list, so adding it here. PS: this is tagged as a feature but only because that was the most applicable of the GitHub issue forms, but it isn't really a feature