The phpunit runner seems to run multiple tests concurrent. This is fine in almost all cases, but for one project I'd like to disable that functionality. Idea is to make it configurable in the .arcconfig (just like configuring the phpunit binary).

Shall I submit a PR for this change? Or am I overlooking something.

ps. In my particular case, I have a heavy test bootstrap code which cannot be run in parallel since it will do some database mutations on first API call (which happens at the first run tests)