tests/stm32: add uart async and blocking flush test.

tests/stm32/src/bin/usart.rs

@@ -33,6 +33,13 @@ async fn main(_spawner: Spawner) {
         let mut buf = [0; 2];
         usart.blocking_read(&mut buf).unwrap();
         assert_eq!(buf, data);
+
+        // Test flush doesn't hang.
+        usart.blocking_write(&data).unwrap();
+        usart.blocking_flush().unwrap();
+
+        // Test flush doesn't hang if there's nothing to flush
+        usart.blocking_flush().unwrap();
     }
 
     // Test error handling with with an overflow error

tests/stm32/src/bin/usart_dma.rs

@@ -51,6 +51,23 @@ async fn main(_spawner: Spawner) {
         assert_eq!(tx_buf, rx_buf);
     }
 
+    // Test flush doesn't hang. Check multiple combinations of async+blocking.
+    tx.write(&tx_buf).await.unwrap();
+    tx.flush().await.unwrap();
+    tx.flush().await.unwrap();
+
+    tx.write(&tx_buf).await.unwrap();
+    tx.blocking_flush().unwrap();
+    tx.flush().await.unwrap();
+
+    tx.blocking_write(&tx_buf).unwrap();
+    tx.blocking_flush().unwrap();
+    tx.flush().await.unwrap();
+
+    tx.blocking_write(&tx_buf).unwrap();
+    tx.flush().await.unwrap();
+    tx.blocking_flush().unwrap();
+
     info!("Test OK");
     cortex_m::asm::bkpt();
 }