Update OpenID certification docs #82

OpenID certification tests have migrated to a new testing system.
It will be better to update the documentation on how to run tests for Dex.

Migration overview: https://openid.net/certification/migration/
New testing process: https://openid.net/certification/connect_op_testing/

Besides, we need to remove test results from the documentation and open the roadmap issue about "basic provider" certification in the dexidp/dex repo.