[Fixes #134] Added Pitest dependency for mutation test

[sohel-79]

Updated pom.xml

Fixes #134

[ascopes]

As posted in #138:

I have just had a play around with pitest locally to make sure this works, and I cannot actually get it to work properly without hanging. For now, I think it is probably worth holding off merging these changes since it is going to cause build issues if it does not complete consistently (which would somewhat defeat the point if having this in the first place).

I've pushed your commits (that I have cherry-picked) onto the branch at https://github.com/ascopes/java-compiler-testing/tree/task/134-pitest though, so when I can get a bit more time to work out what is actually happening with this, it can be merged.

Thanks for the help on this :-)