bring RP logout tests inline with OP logout tests wrt. requirement on id_token_hint

[zandbelt]

id_token_hint should be present when post_logout_redirect_uri is present to expose the same behaviour as the OP tests

• https://bitbucket.org/openid/connect/issues/1032/rp-initiated-logout-proposal-for-client_id#comment-51373773
• https://bitbucket.org/openid/connect/issues/1071

[rohe]

Updated oidctest to do this. It should eventually get into pyoidc but perhaps not before the draft has been changed.

[zandbelt]

that should be in the latest release then https://github.com/openid-certification/oidctest/releases/tag/v1.2.3, I will verify shortly