Allow switching to Bakerloo environment with the AFP_TESTNET env var

[szemate]

No description provided.

[aiman]

Is is worth documenting that AFP_TESTNET will not override AFP_EXCHANGE_URL, as a user may set both in an inconsistent manner, and there is no check for inconsistency. Or is this unnecessary?

[szemate]

Is is worth documenting that AFP_TESTNET will not override AFP_EXCHANGE_URL, as a user may set both in an inconsistent manner, and there is no check for inconsistency. Or is this unnecessary?

It's a good point, and it also applies to the contract addresses.