Add tests for new orderbook endpoints

[porkbrain]

Some of the new orderbook endpoints might have bugs in what they return. For example, creating a safe, then depositing and then listing an NFT returns first created object as trade object. However, it creates also a Safe and OwnerCap. There might therefore now be additional logic which decides which of these objects is which, or we should remove the output type altogether and let the user to figure it out themselves.