tests/avocado: introduce alpine virt test for CI

tests/avocado: introduce alpine virt test for CI

The boot_linux tests download and run a full cloud image boot and
start a full distro. While the ability to test the full boot chain is
worthwhile it is perhaps a little too heavy weight and causes issues
in CI. Fix this by introducing a new alpine linux ISO boot in
machine_aarch64_virt.

This boots a fully loaded -cpu max with all the bells and whistles in
31s on my machine. A full debug build takes around 180s on my machine
so we set a more generous timeout to cover that.

We don't add a test for lesser GIC versions although there is some
coverage for that already in the boot_xen.py tests. If we want to
introduce more comprehensive testing we can do it with a custom kernel
and initrd rather than a full distro boot.

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Message-Id: <20221117172532.538149-10-alex.bennee@linaro.org>