Add tests for upcoming difficulty bomb push fork

[marioevz]

As the title says, the following bomb push will happen in the upcoming weeks and will likely be a 700,000 block push.
Appropriate tests will be added to this repository.

[jochem-brouwer]

Is there an EIP or spec somewhere? And a fork name?

[marioevz]

None that I know of yet, but will keep this issue updated once I have all the final values and the EIP number.
@timbeiko do you know if we have an EIP yet ? Thanks.

[timbeiko]

See the spec for the fork here: ethereum/execution-specs#537

I think unless someone has an objection, it's unlikely to change, but want to wait until wednesday to merge.

[marioevz]

Added PR #1043 for this.