SSO: redirect to public URL before setting cookies (#9436)

... otherwise, we don't get the cookie back.

tests/rest/client/v2_alpha/test_auth.py

@@ -161,7 +161,11 @@ class UIAuthTests(unittest.HomeserverTestCase):
 
     def default_config(self):
         config = super().default_config()
-        config["public_baseurl"] = "https://synapse.test"
+
+        # public_baseurl uses an http:// scheme because FakeChannel.isSecure() returns
+        # False, so synapse will see the requested uri as http://..., so using http in
+        # the public_baseurl stops Synapse trying to redirect to https.
+        config["public_baseurl"] = "http://synapse.test"
 
         if HAS_OIDC:
             # we enable OIDC as a way of testing SSO flows