tests/acceptance: Test the Arduino MEGA2560 board

tests/acceptance: Test the Arduino MEGA2560 board

The test is based on
https://github.com/seharris/qemu-avr-tests/tree/master/free-rtos/Demo
demo which. If working correctly, prints 'ABCDEFGHIJKLMNOPQRSTUVWX' out.
it also demostrates that timer and IRQ are working

As the path name demonstrates, the FreeRTOS tests target a
board based on a ATMega2560 MCU. We have one, the Arduino
MEGA2560.

Complementary documentation:

https://feilipu.me/2012/01/15/ethermega-arduino-mega-2560-and-freertos/
https://feilipu.me/2015/11/24/arduino_freertos/ (see 'Compatibility')

Signed-off-by: Michael Rolnik <mrolnik@gmail.com>
Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
[rth: Squash multiple avocado fixups from f4bug]
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Aleksandar Markovic <aleksandar.m.mail@gmail.com>
Tested-by: Richard Henderson <richard.henderson@linaro.org>
Tested-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Acked-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>