Remove lock on FS and run tests in parallel

[JekaMas]

First of all, thank you for your project!

We have just started using mutation test and the very first question is about performance. It look quite a while to run tests. I believe is it all about FS and that we can run only one test at a time, and have to perform quite a few FS operations.

Have you tried anything in that direction? Virtual, layered FSs or lightweight docker image?