parallel execution

aloha!

i have seen [this comment](https://github.com/status-im/nim-unittest2/blob/d4cbd84848391d5085d9b51e8bb12fcd42dd8fa3/unittest2.nim#L76C1-L78C60) and i am interested what was exactly safety concern? 

would it be possible to reintroduce this feature, were developers would enable it optionally?

