Add test for the genesis block

[joelamouche]

Should cover polkadot-evm/frontier#78
"Using Moonbeam Alphanet, which is using frontier, I discovered this same issue. I'm able to get the block 0 using the eth_getBlockByNumber RPC call, but using then the hash that appears there to get the block again with eth_getBlockByHash, it results in null."

• fetch genesis block by number
• fetch genesis block by hash

And also add those to frontier