chore(PocketIC): retry in PocketIC bitcoin integration tests

rs/pocket_ic_server/tests/bitcoin_integration_tests.rs

@@ -140,9 +140,16 @@ rpcauth=ic-btc-integration:cdf2741387f3a12438f69092f0fdad8e$62081498c98bee09a0dc
 
     // `n` must be more than 100 (Coinbase maturity rule) so that the reward for the first block can be sent out
     let mut n = 101;
-    btc_rpc
-        .generate_to_address(n, &Address::from_str(&bitcoin_address).unwrap())
-        .unwrap();
+    // retry generating blocks until the bitcoind is up and running
+    loop {
+        match btc_rpc.generate_to_address(n, &Address::from_str(&bitcoin_address).unwrap()) {
+            Ok(_) => break,
+            Err(bitcoincore_rpc::Error::JsonRpc(_)) => {
+                std::thread::sleep(std::time::Duration::from_millis(100));
+            }
+            Err(err) => panic!("Unexpected error when talking to bitcoind: {}", err),
+        }
+    }
 
     let reward = 50 * 100_000_000; // 50 BTC