Add onRetry for setting JVM options for fork differently

[U1F984]

Fixes #205, #206, #209, #271.

Going by my last submitted PR I don't expect this one to be accepted either, so I've not bothered to add any tests/docs.

[pshevche]

@U1F984 , thank you for looking into it. I am not convinced that we'll go down this route. It does not cover some of the use cases from #271, like "Print a message on successful retry". Furthermore, we need to think more about the semantics of this callback: should it be invoked once, meaning that the test retry plugin kicked in, or on every retry? I am sorry it takes us so long to get to this topic. We still plan to investigate it, but we'll need to see first how it ranks against other priorities.