This change removes the use of hardcoded port values for the
idp-fixture in favor of the mapped ephemeral ports. This should prevent
failures due to port conflicts in CI.
The change replaces the Vagrant box based fixture with a fixture
based on docker compose and 2 docker images, one for an openldap
server and one for a Shibboleth SAML Identity Provider.
The configuration of both openldap and shibboleth is identical to
the previous one, in order to minimize required changes in the
tests