Design & implement reactor style tests for full Zug-compatible rewards process

[SaiProServ]

Follow up to #3914 and #4289

[rafal-ch]

Plan of attack:

1. Refactor out effect_builder from the reward calculation functions in order to be able to unit test them
2. Create a sunny-day mockreactor-like test to check if network is emitting rewards info on switch block
3. (but can happen in parallel with the two above) brainstorm with @fizyk20 and @AlexanderLimonov about potential testing scenarios (for example: validator going down or missing finality signatures, etc.)

[AlexanderLimonov]

Noted