[CI] Add OpenLDAP integration tests

This PR sets up an additional set of test environments that installs OpenLDAP and runs special integration tests that connects against that OpenLDAP server.

This was based off of similar work in github/github-ldap: https://github.com/github/github-ldap/pull/48.

Will flesh out the integration tests today, still more to come.

cc @jch @schaary 

@jch were you thinking of overriding `run` to do nothing in that case?

Check out 26ebfff2ef5f73d6885676a5aa5e18c6167a6532.