[Feature Request] Add mock mixins

[diegotori]

It's pretty much impossible to mock any of the Chopper related classes due to the recent Dart 3 upgrade, since they're now using either base or final keywords in their class declarations.

Based on the closed PR #479, there was an attempt to add this, but it was tightly coupled with mockito, which may not apply to developers such as myself who don't use it (I happen to use mocktail).

As a result, I propose that we make available (@visibleForTesting of course) mixins that can enable developers to mock out any Chopper related component based on this suggestion from the Dart Team.

I will submit a PR to this effect very shortly.