Refactor code for calculating registration flows (#6106)

because, frankly, it looked like it was written by an axe-murderer.

This should be a non-functional change, except that where `m.login.dummy` was
previously advertised *before* `m.login.terms`, it will now be advertised
afterwards. AFAICT that should have no effect, and will be more consistent with
the flows that involve passing a 3pid.

tests/test_terms_auth.py

@@ -28,6 +28,21 @@ from tests import unittest
 class TermsTestCase(unittest.HomeserverTestCase):
     servlets = [register_servlets]
 
+    def default_config(self, name="test"):
+        config = super().default_config(name)
+        config.update(
+            {
+                "public_baseurl": "https://example.org/",
+                "user_consent": {
+                    "version": "1.0",
+                    "policy_name": "My Cool Privacy Policy",
+                    "template_dir": "/",
+                    "require_at_registration": True,
+                },
+            }
+        )
+        return config
+
     def prepare(self, reactor, clock, hs):
         self.clock = MemoryReactorClock()
         self.hs_clock = Clock(self.clock)
@@ -35,17 +50,8 @@ class TermsTestCase(unittest.HomeserverTestCase):
         self.registration_handler = Mock()
         self.auth_handler = Mock()
         self.device_handler = Mock()
-        hs.config.enable_registration = True
-        hs.config.registrations_require_3pid = []
-        hs.config.auto_join_rooms = []
-        hs.config.enable_registration_captcha = False
 
     def test_ui_auth(self):
-        self.hs.config.user_consent_at_registration = True
-        self.hs.config.user_consent_policy_name = "My Cool Privacy Policy"
-        self.hs.config.public_baseurl = "https://example.org/"
-        self.hs.config.user_consent_version = "1.0"
-
         # Do a UI auth request
         request, channel = self.make_request(b"POST", self.url, b"{}")
         self.render(request)