Create validator Ethereum address during simulation tests in a correct way

[rach-id]

As of #137 (comment), we're creating a new Ethereum address using the hex representation of the simulated validator account address.
This is currently used as a hack to make the tests pass without having to resort to a massive change: providing a list of Ethereum addresses for each validator that can be used when creating it.
This is an issue because, at first, we were generating a random Ethereum address on each run. The deterministic simulation tests end up with a different app hash on each run. This breaks the test. Thus, we needed a more deterministic way to create an Ethereum address for the validator in question, that would be the same on each run.
Luckily, we're able to use the validator account address to derive an Ethereum address without having to rewrite the whole simulation and still make the tests pass.