Use Symfony Process to spawn subprocesses in PhpUnitTestRunner

Problem/Motivation

As seen in #3417066-13: Drop Symfony PHPUnit-bridge dependency and in #3217904: [meta] Support PHPUnit 10 in Drupal 10, in PHPUnit 10 the PHPUnit Unit pipeline job gets stuck.

This seems to be overcome by using Symfony Process in lieu of exec() to spawn subprocesses executing individual tests.

Replacing that also for PHPUnit sounds like an healthy step - it will also allow to set a process timeout to kill tests taking too long and prevent idle bot runs.

Proposed resolution

Adopt Symfony Process to execute PHPUnit subprocesses.

Merge request link

Remaining tasks

User interface changes

API changes

Data model changes

Release notes snippet