Some tests are currently wrapped with
`if Process.respond_to?(:fork)`
but they're still executed on JRuby. The problem is that ForkTracker
unconditionally patches `#fork` so `Process` does indeed
`respond_to? :fork`.
Fix it by installing the patch conditionally with the same check.
We hook into Kernel#fork and Process#fork, when they are invoked
we can trigger all the registered callbacks.
There is also a `check!` method that can be called to very cheaply
detect if the process was forked.
Michael Grosser