Add QA test for SAML test button

What

• Test that SAML test button returns from the identity provider to the settings page instead of elsewhere, and that a success message is shown.
• Potentially test that details of the error are shown when the identity provider is misconfigured.

Why

At the moment we're testing this with OmniAuth::Strategies::GroupSaml#mock_redirect_relay_state, which is messy and mimics too much of the internals from OmniAuth::Strategies::SAML#additional_params_for_authn_request and OneLogin::RubySaml::Authrequest.