Add metamask compliance tests

[pnowosie]

We agreed on slack that testing metamask compliance regarding structural hash will be more maintainable in this repo than in omisego/elixir-omg.

Here is original message:

---

In elixir-omg we have metamask 🦊 structural hash compliance test
https://github.com/omisego/elixir-omg/blob/4d0e8b4c60efb2072dab1c8179c1644679a243f4/apps/omg/test/omg/typed_data_hash_test.exs#L178
which are just hard-coded hashes we've generated once playing with browser extension

I'm thinking of moving the test to omg-js because 🦊 's eth-sig-util looks promising https://github.com/MetaMask/eth-sig-util#typedsignaturehash-typeddata and it should be more natural to combine it in JS native env 🙂

My idea is as follows:
• omg-js talking to ch-ch (I've heard we have / planning e2e tests) so we can verify sigs js -> elixir
• omg-js verifies signatures against metamask with above eth-sig-util: js -> metamask
• elixir-omg integration tests or omg-js e2e talks to contract so we have elixir/js -> solidity verification

[pdobacz]

For future reference: the PR linked just above (omgnetwork/elixir-omg#1005) cuts the metamask-compliance tests out of elixir-omg, in case we need that old code for anything