Allow ASB to be configured with max BTC buy amount

This will make it easier to also configure the CLI to display an appropriate max amount the user has to deal with.

swap/tests/testutils/mod.rs

@@ -394,7 +394,7 @@ where
         alice_monero_wallet.clone(),
         alice_db,
         fixed_rate::RateService::default(),
-        alice_starting_balances.xmr,
+        bitcoin::Amount::ONE_BTC,
     )
     .unwrap();