[php-src] Issue #20980: Test parallellization uses a stupid strategy

Issue: https://github.com/php/php-src/issues/20980
Author: ndossche

### Description

When running ./run-test.php with the -j argument, all the tests are scheduled to the workers before even running them. This means that if you have a worker which finishes all its tests quickly it'll sit idle even if there are many more other tests to run. One can observe this often because at the end of a test run the number of concurrent working test runners drops quickly.

### PHP Version

```plain
8.4+
```

### Operating System

N/A