AP_HAL: add delay to UART_test example

For AC3.5 and higher version, serial uartA-USBconsole cannot work. Maybe the code before "setup" has been changed. Ensure that the uartA can be initialized

libraries/AP_HAL/examples/UART_test/UART_test.cpp

@@ -31,6 +31,9 @@ void setup(void)
     /*
       start all UARTs at 57600 with default buffer sizes
     */
+
+    hal.scheduler->delay(1000); //Ensure that the uartA can be initialized
+
     setup_uart(hal.uartA, "uartA");  // console
     setup_uart(hal.uartB, "uartB");  // 1st GPS
     setup_uart(hal.uartC, "uartC");  // telemetry 1