Support for mocking Process.clock_gettime via Timecop

[chrisbutcher]

It would be great to be able to freeze and travel time returned by this method.

https://ruby-doc.org/core-2.4.2/Process.html#method-c-clock_gettime

More on monotonic clocks: https://www.softwariness.com/articles/monotonic-clocks-windows-and-posix/

[evan-stripe]

(It'd be helpful if this worked on CLOCK_REALTIME as well as CLOCK_MONOTONIC - we're moving some particularly performance-sensitive code to use Process.clock_gettime(Process::CLOCK_REALTIME) instead of Time.now to avoid an object allocation)

[joshuacronemeyer]

i've got to spend some time with @wishdev's PR, play around with it and do some tests but think this will be a nice new feature to have in our next release.