tests/avocado: update aarch64_virt test to exercise -cpu max

The Fedora 29 kernel is quite old and importantly fails when running
in LPA2 scenarios. As it's not really exercising much of the CPU space
replace it with a custom 5.16.12 kernel with all the architecture
options turned on. There is a minimal buildroot initramfs included in
the kernel which has a few tools for stress testing the memory
subsystem. The userspace also targets the Neoverse N1 processor so
would fail with a v8.0 cpu like cortex-a53.

While we are at it move the test into its own file so it can have an
assigned maintainer.

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Acked-by: Richard Henderson <richard.henderson@linaro.org>
Tested-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20220419091020.3008144-2-alex.bennee@linaro.org>