Test that both parties refund if Alice does not redeem

Also:

- Move generator functions to `alice` and `bob` modules. This makes
using `tracing` a lot easier, since the context of the file name let's
us differentiate between Alice's and Bob's generator logs more
clearly.
- Accept 0 confirmations when watching for the Monero lock
transaction. This should eventually be configured by the application,
but in the tests it's making things unexpectedly slower.

xmr-btc/src/bitcoin.rs

@@ -190,6 +190,16 @@ pub trait WatchForRawTransaction {
     async fn watch_for_raw_transaction(&self, txid: Txid) -> Transaction;
 }
 
+#[async_trait]
+pub trait BlockHeight {
+    async fn block_height(&self) -> u32;
+}
+
+#[async_trait]
+pub trait TransactionBlockHeight {
+    async fn transaction_block_height(&self, txid: Txid) -> u32;
+}
+
 pub fn recover(S: PublicKey, sig: Signature, encsig: EncryptedSignature) -> Result<SecretKey> {
     let adaptor = Adaptor::<Sha256, Deterministic<Sha256>>::default();
 
@@ -200,3 +210,16 @@ pub fn recover(S: PublicKey, sig: Signature, encsig: EncryptedSignature) -> Resu
 
     Ok(s)
 }
+
+pub async fn poll_until_block_height_is_gte<B>(client: &B, target: u32)
+where
+    B: BlockHeight,
+{
+    loop {
+        if client.block_height().await >= target {
+            return;
+        }
+
+        tokio::time::delay_for(std::time::Duration::from_secs(1)).await;
+    }
+}