tests: do not export test helping functions or mocks

## Task summary

We should be more careful about polluting our Go packages API with exported mocks and test helping functions. In that case it is better to keep those unexported and duplicate the code.

This would also allow to decouple packages more easily without having to struggle with test cycle dependencies.

This issue should move all our mocks generated code and helping functions in `_test.go` files in the package where they are needed.