complete: add SetExitFunc() to override os.Exit in tests

[charlievieth]

This eliminates the need to use the unsupported bou.ke/monkey package, which fails to build on darwin/arm64. A better solution might be to remove the use of os.Exit entirely since it's only called with os.Exit(0) and exiting the program should be the responsibility of the caller to Complete.

[posener]

I think that this breaks the current package API. No?