Add tests for consensus on valid G2 points

[blacktemplar]

The specs describe in https://github.com/ethereum/eth2.0-specs/blob/dev/specs/bls_signature.md#g2-points exactly the conditions when a G2 point is valid.

I think a potential consensus attack on the eth2.0 network could be creating deposits which have invalid signatures based on those rules. So it is very important to not break consensus that all clients recognize the same signatures (compressed as G2 points) as valid/invalid.

Therefore I propose adding tests which test the conditions specified in https://github.com/ethereum/eth2.0-specs/blob/dev/specs/bls_signature.md#g2-points. As far as I can see such tests are not existent at the moment (whenever we test invalid signatures for deposit we use the signature with all 0s).

[protolambda]

@JustinDrake What do you think? Does it make sense to prioritize this, even though the current hash_to_G2 is non-final/unsecure?

[JustinDrake]

A test suite is being developed alongside the standardisation effort. I expect we can merge those tests in when the standardisation is a bit more mature.