Ha, yes, Jude discovered bitcoind’s regtest earlier and I totally forgot that the integration tests now use Bitcoin’s regtest. I think one of the reasons we assumed testnet is because testnet on bitcoin is much more widely known that regtest, where as regtest is a lot more powerful and better for testing. Quoting from bitcoin.org:
Bitcoin Core’s regression test mode (regtest mode) lets you instantly create a brand-new private block chain with the same basic rules as testnet—but one major difference: you choose when to create new blocks, so you have complete control over the environment.