f No use continuing if we never subscribed

So try again after some time and panic if it still fails

lightning-transaction-sync/tests/integration_tests.rs

@@ -64,19 +64,23 @@ fn generate_blocks_and_wait(num: usize) {
 }
 
 fn wait_for_block(min_height: usize) {
-	let mut cur_height = get_electrsd().client.block_headers_subscribe().map_or(0, |header| header.height);
+	let mut header = match get_electrsd().client.block_headers_subscribe() {
+		Ok(header) => header,
+		Err(_) => {
+			std::thread::sleep(Duration::from_secs(3));
+			get_electrsd().client.block_headers_subscribe().expect("failed to subscribe to block headers")
+		}
+	};
 
 	loop {
-		if cur_height >= min_height {
+		if header.height >= min_height {
 			break;
 		}
-		let header = exponential_backoff_poll(|| {
+		header = exponential_backoff_poll(|| {
 			get_electrsd().trigger().expect("failed to trigger electrsd");
 			get_electrsd().client.ping().expect("failed to ping electrsd");
 			get_electrsd().client.block_headers_pop().expect("failed to pop block header")
 		});
-
-		cur_height = header.height;
 	}
 }