On WAYF functional testing endpoint post data to test

[phavekes]

This issue is imported from pivotal - Originaly created at Apr 3, 2019 by bstrooband

Currently the data from the WAYF is posted to a bogus url when clicking on an Idp. It would be very valuable to post to the endpoint itself to output a JsonResponse to be able to validate the data posted.

This could be done with minimal effort while increasing the test coverage a lot.