Set shouldRunSequentially to false by default or just remove runSynchronized

[mkurz]

It's all explained here:

• Parallel test case execution #12026

There is no real need anymore to run tests sequentially anymore, now that we use random ports for testing and we plan to remove the global state as well anyway:

• Remove Play app global state #9263
• Remove global state #11838

Also optionalGlobalLock method and the mutex can be removed I guess.

I am pretty sure though this is blocked by

• Correctly shutdown current active LoggerConfigurator on app (re)load in DEV mode #10939

Edit: I made some notes locally once about this, where I thought we might introduce a sys property which can be used to enable/disabled parallelism again, not sure what I was thinking about back then, but this was while I was working on the parallel port thing...