erc20: Add USDC testnet support.

[JoeGruffins]

USDC appears to have a testnet contract at https://goerli.etherscan.io/address/0x07865c6e87b9f70255377e024ace6630c1eaa37f which is being actively used and a faucet at https://usdcfaucet.com/ (untested) Making it a good candidate for testing.

After #1622 goes in it should be straightforward to add. Adding includes deploying our erc20 swap contract on the goerli testnet with the token address as the USDC testnet address. We can also point the harness testnet tests at that new contract and get them working.

If for some reason a different coin is preferable, please comment. We could even deploy our own token, but I don't think that's necessary unless the faucet turns out to not work very well.

[JoeGruffins]

Will work on adding.

[JoeGruffins]

https://goerli.etherscan.io/address/0x9e493d3766989e701797b9371682b7b94fd8af9c#code

Curiously, deploying with geth didn't need/want as much zero padding for the token address as our harness.