Integration with official Deposit Contract

[ongrid]

Now IValidatorRegistration interface and ValidatorRegistrationMock are synthetic. To prove compatibility with real-world environment, we should switch to official Ethereum 2 DepositContract and use it in all the test scenarios.

Note: Since there is a lot of math and costly operations (including iterative Merkle Tree walks) inside the real deposit call we can get new gas estimates and the numbers may depend on the Deposit contract state. Probably we'll need to test it at scale.

Related issue: #82

[skozin]

Actually, the current gas estimations include corrections for the difference between the real and mock deposit contracts in terms of gas costs (the mock one is much more costly). Also, we cannot hope to reproduce real deposit contract state that we’ll see in the full-scale testnet, so it’s more reliable to just use the numbers we see in the current deployed public contract instance, just like we’re doing right now.

Nevertheless, integrating the real contract code is a good thing. But we should make sure that readability of the scenario tests doesn’t suffer as a result.