Add secondary while loop to serai/client's test runner

There's a failing CI run on a node which booted, yet didn't create a genesis
yet. Apparently, the RPC is potentially accessible before the chain is in.

This attempts to resolve that.

substrate/serai/client/tests/runner.rs

@@ -106,6 +106,9 @@ macro_rules! serai_test {
           while Serai::new(URL).await.is_err() {
             tokio::time::sleep(Duration::from_secs(1)).await;
           }
+          while Serai::new(URL).await.unwrap().get_latest_block_hash().await.is_err() {
+            tokio::time::sleep(Duration::from_secs(1)).await;
+          }
           command
         };