In the Merkle Proof 2017 schema definition, the
chain string can be one of the values defined here:
The question is, what chain does ethereumTestnet refers to?
There are multiple Ethereum testnets like Ropsten (already has a chain value called
ethereumRopsten), Rinkeby, Goerli…
PD1: The reference verification library does not use this chain value in their
PD2: Ask it here because the repository in GitHub has no issues section.