PoC BeaconChainController tests

[zilm13]

PR Description

Looking on a way we could test BeaconChainController in order to:

• verify event channel and other types of subscriptions
• verify call order to avoid regression when something is null at the place it's used

Fixed Issue(s)

Documentation

• I thought about documentation and added the doc-change-required label to this PR if updates are required.

Changelog

• I thought about adding a changelog entry, and added one if I deemed necessary.