ByzantiumToConstantinopleFixAt5 transition does not check that it's actually a transition to Petersburg

[veox]

Fallout from ethereum/py-evm#1858 (review).

py-evm was erroneously transitioning to Constantinople, not Petersburg (a copy-paste error).

The test that checks the transition doesn't seem to have anything Petersburg-specific, so the test was still passing.

Not sure how to address this.

[winsvega]

You right. the initial test was done to see the difficulty bomb change from Byzantium to Constantinople
and the mining rewards + uncle rewards.

the solution is to add more sstore operations to this test

[qbzzt]

Added in #768