Mock child_process.exec for tests

[sebmck]

I want to do something similar to what we did with the network cache. Cache stderr/stdout and file system events that the child process performs and then replay them back.

[kentaromiura]

@cpojer this looks like something that would be possible to make generic enough to be reused with different APIs, should we consider adding to jest something similar to snapshot but for mocked functions?

I'm thinking at something likejest.memoizeFn(realFn, [optional-mock-name]) where the mocked function would check in a local cache if the result for the provided arguments exists, if not will run the original function and store the result.
The artifact here imho doesn't even have to be pretty-printed making it easier to implement.