feat request: allow mocking of internal functions

[asafyish]

It would be very useful to mock internal/private functions.
I have a public/external function that calls a bunch of internal functions and I would like to change those internal functions behaviour.

[smartcontracts]

Hi, unfortunately mocking internal functions can't be supported until we refactor the code to integrate at a deeper level. We probably have to modify the ethereumjs-VM entirely for this to work.