Fix module sort comparator and drop Drupal 8/9 dead code in autoloader

[mglaman]

Part 5 of 9 in the legacy-code audit stack (on top of #1042).

What changed

• Module sort comparator fixed. The usort callback in DrupalAutoloader returned 10 or 0 and ignored its second operand, so it was not a valid comparator and the order of test modules was implementation-defined. On PHP 8 it did not sort test modules last at all. Replaced with a valid comparator that reliably loads _test extensions after regular ones.
• Why the sort exists. Test modules stub functions from their parent module behind function_exists() guards. If the test module's .module file loads first, the parent's unconditional declaration is a compile error that loadAndCatchErrors() cannot intercept. This was the original blazy_test fix from 2019. The comment now says so.
• Removed the Drupal 8/9-only PhpUnit8\ClassWriter shim (the class was removed in Drupal 10) and the Drush 8 directory-depth branch (composer requires drush ^11 || ^12 || ^13).

Known consequence

Service definitions are last-writer-wins by ID. With the ordering now deterministic, a test module that redefines a production service ID always wins in the service map, where before the winner depended on filesystem order. Core's own collisions are against core.services.yml, which is processed first regardless, and the pattern is rare in contrib. Tracked as a follow-up together with a path-based test-module check, since _test in the name misses test extensions under tests/ directories and catches real modules like ab_tests.

Testing

Full suite, self-analysis, and phpcs are green.

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