Create Beacon API/P2P Tests Simulator

[marioevz]

It has been suggested that there have been simple bugs in the P2P interface between beacon clients, which is basically clients not following the spec.

I'm opening this issue to start the work on P2P beacon interface tests.

• eth2/beacon/p2p containing tests that verify https://github.com/ethereum/consensus-specs/blob/dev/specs/phase0/p2p-interface.md, and subsequent specifications of newer forks
• eth2/beacon/api containing tests that verify https://github.com/ethereum/beacon-APIs