Add specs for Process._fork

[HeroProtagonist]

Relates to #923

Process._fork is added. This is a core method for fork(2).
Do not call this method directly; it is called by existing
fork methods: Kernel.#fork, Process.fork, and IO.popen("-").
Application monitoring libraries can overwrite this method to
hook fork events. [Feature #17795]

[eregon]

Thank you!