automation: add tests artifacts

automation: add tests artifacts

Some tests (soon to come) will require pre-built binaries to run, such
as the Linux kernel binary. We don't want to rebuild the Linux kernel
for each gitlab-ci run: these builds should not be added to the current
list of build jobs.

Instead, create additional containers that today are built and uploaded
manually, but could be re-built automatically. The containers build the
required binarires during the "docker build" step and store them inside
the container itself.

gitlab-ci will be able to fetch these pre-built binaries during the
regular test runs, saving cycles.

Add two tests artifacts containers:
- one to build the Linux kernel ARM64
- one to create an Alpine Linux ARM64 rootfs for Dom0

Signed-off-by: Stefano Stabellini <stefano.stabellini@xilinx.com>
Acked-by: Wei Liu <wl@xen.org>