test(cluster): add startup wait to proxy_write test

The  test panics with connection refused
because it opens a database connection to replica0 before it is
fully started.  Add an HTTP-poll loop (matching the pattern used in
replication.rs and other cluster tests) to wait for the replica's
user API endpoint to become responsive before proceeding.

Author: aredridel

libsql-server/tests/cluster/mod.rs

@@ -93,6 +93,12 @@ fn proxy_write() {
     make_cluster(&mut sim, 1, true);
 
     sim.client("client", async {
+        let client = Client::new();
+        // wait for replica to start up before connecting
+        while client.get("http://replica0:8080/").await.is_err() {
+            tokio::time::sleep(Duration::from_millis(100)).await;
+        }
+
         let db =
             Database::open_remote_with_connector("http://replica0:8080", "", TurmoilConnector)?;
         let conn = db.connect()?;