tests/docker: add podman support

Allow to specify the container engine to run with ENGINE variable.

By default, ENGINE=auto and will select either podman or docker.

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>