Fix that `flutter test` does not understand `concurrency` (#125942)

Close https://github.com/flutter/flutter/issues/125940

I will add tests if this PR looks roughly OK :)

The fix mainly mimics https://github.com/flutter/flutter/pull/115160 - just remove the default argument.

p.s. I ran into this bug when wanting to set concurrency in my dart_test.yaml for one set of my tests which I need to be executed without parallalization.