Add a mandatory --change-address parameter to buy-xmr

Fixes #513.

swap/tests/harness/mod.rs

@@ -434,6 +434,7 @@ impl BobParams {
             self.env_config,
             handle,
             self.monero_wallet.get_main_address(),
+            self.bitcoin_wallet.new_address().await?,
             btc_amount,
         );