tests/docker: Refresh APT cache before installing new packages on Debian

Since docker caches the different layers, updating the package
list does not invalidate the previous "apt-get update" layer,
and it is likely "apt-get install" hits an outdated repository.

See commit beac6a98 and
https://docs.docker.com/develop/develop-images/dockerfile_best-practices/#apt-get



Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-Id: <20190723141528.18023-1-philmd@redhat.com>
[AJB: manually applies and fixed up]
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>