rust tests: Wait between chain tip polling even when successful

Without a delay, this occasionally fails to catch up after the 120 attempts.

subprojects/gdk_rust/tests/test_session.rs

@@ -188,9 +188,9 @@ pub fn setup(
             }
             Err(e) => {
                 warn!("{:?}", e);
-                thread::sleep(Duration::from_millis(500));
             }
         }
+        thread::sleep(Duration::from_millis(500));
     };
     let electrs = RawClient::new(&electrs_url).unwrap();
     info!("done creating electrs client");