improve rp-response_mode-form_post-error

[zandbelt]

As a followup on #119 the following improvements should be made to rp-response_mode-form_post-error:

• the test description should state the conditions HOW to trigger this error, (max_age, prompt)
• when testing without a state parameter the suite returns from the authorization endpoint with the text Unrecoverable error in the server. response

  Traceback (most recent call last):
  File "/usr/local/lib/python3.6/dist-packages/CherryPy-8.9.1-py3.6.egg/cherrypy/_cprequest.py", line 670, in respond
  response.body = self.handler()
  File "/usr/local/lib/python3.6/dist-packages/CherryPy-8.9.1-py3.6.egg/cherrypy/lib/encoding.py", line 220, in __call__
  self.body = self.oldhandler(*args, **kwargs)
  File "/usr/local/lib/python3.6/dist-packages/CherryPy-8.9.1-py3.6.egg/cherrypy/_cpdispatch.py", line 60, in __call__
  return self.callable(*self.args, **self.kwargs)
  File "/usr/local/lib/python3.6/dist-packages/oidctest-0.7.3-py3.6.egg/oidctest/cp/op.py", line 216, in index
  resp = op.authorization_endpoint(kwargs)
  File "/usr/local/lib/python3.6/dist-packages/oidctest-0.7.3-py3.6.egg/oidctest/rp/provider.py", line 451, in authorization_endpoint
  "state": _req['state']
  KeyError: 'state'