修复无法重复使用finsh命令device_test进行驱动测试

1. 该问题是打开设备后, 出错或者成功后没有关闭设备而导致.

examples/test/device_test.c

@@ -449,11 +449,13 @@ static rt_err_t _block_device_test(rt_device_t device)
             }
         } /* step 5: multiple sector speed test */
 
+        rt_device_close(device);
         return RT_EOK;
     }// device can read and write.
     else
     {
         // device read only
+        rt_device_close(device);
         return RT_EOK;
     }// device read only
 
@@ -466,6 +468,7 @@ __return:
     {
         rt_free(write_buffer);
     }
+    rt_device_close(device);
     return RT_ERROR;
 }