tests/acceptance: Add a test for the sun4u sparc64 machine

tests/acceptance: Add a test for the sun4u sparc64 machine

We can use the image from the advent calendar 2018 to test the sun4u
machine. It's not using the "QEMU advent calendar" string, so we can
not use the do_test_advcal_2018() from boot_linux_console.py, thus
let's also put it into a separate file to also be able to add an
entry to the MAINTAINERS file.

Message-Id: <20200704173519[email protected]>
Tested-by: Mark Cave-Ayland <[email protected]>
Tested-by: Philippe Mathieu-Daudé <[email protected]>
Reviewed-by: Philippe Mathieu-Daudé <[email protected]>
Signed-off-by: Thomas Huth <[email protected]>