Enable concurrent phpunit

Changes are straightforward and pipeline time is down from ~1 hour to ~10 mins.

Test failures are solved by 🐛 Fix Optional parameter X declared before required parameter Y Active - interesting that they fail the pipeline with concurrent enabled.

liam morland → made their first commit to this issue’s fork.

I have the impression that concurrent tests are flaky. I've tried to merge it a couple of times and each time, something different fails. If we manage to get it to merge, we may face ongoing test flakiness.

That's really strange, I see what you mean but I don't understand how that could be the case considering tests are self isolated (each test case spins up a fresh Drupal installation and tears itself down)

Some suggestions in this slack thread about how to resolve this https://drupal.slack.com/archives/CGKLP028K/p1727992456929179?thread_ts=...