Refactor OIDC tests to better mimic an actual OIDC provider. (#13910)

This implements a fake OIDC server, which intercepts calls to the HTTP client.
Improves accuracy of tests by covering more internal methods.

One particular example was the ID token validation, which previously mocked.

This uncovered an incorrect dependency: Synapse actually requires at least
authlib 0.15.1, not 0.14.0.

tests/federation/test_federation_client.py

@@ -12,13 +12,10 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
-import json
 from unittest import mock
 
 import twisted.web.client
 from twisted.internet import defer
-from twisted.internet.protocol import Protocol
-from twisted.python.failure import Failure
 from twisted.test.proto_helpers import MemoryReactor
 
 from synapse.api.room_versions import RoomVersions
@@ -26,10 +23,9 @@ from synapse.events import EventBase
 from synapse.rest import admin
 from synapse.rest.client import login, room
 from synapse.server import HomeServer
-from synapse.types import JsonDict
 from synapse.util import Clock
 
-from tests.test_utils import event_injection
+from tests.test_utils import FakeResponse, event_injection
 from tests.unittest import FederatingHomeserverTestCase
 
 
@@ -98,8 +94,8 @@ class FederationClientTest(FederatingHomeserverTestCase):
 
         # mock up the response, and have the agent return it
         self._mock_agent.request.side_effect = lambda *args, **kwargs: defer.succeed(
-            _mock_response(
-                {
+            FakeResponse.json(
+                payload={
                     "pdus": [
                         create_event_dict,
                         member_event_dict,
@@ -208,8 +204,8 @@ class FederationClientTest(FederatingHomeserverTestCase):
 
         # mock up the response, and have the agent return it
         self._mock_agent.request.side_effect = lambda *args, **kwargs: defer.succeed(
-            _mock_response(
-                {
+            FakeResponse.json(
+                payload={
                     "origin": "yet.another.server",
                     "origin_server_ts": 900,
                     "pdus": [
@@ -269,8 +265,8 @@ class FederationClientTest(FederatingHomeserverTestCase):
 
         # We expect an outbound request to /backfill, so stub that out
         self._mock_agent.request.side_effect = lambda *args, **kwargs: defer.succeed(
-            _mock_response(
-                {
+            FakeResponse.json(
+                payload={
                     "origin": "yet.another.server",
                     "origin_server_ts": 900,
                     # Mimic the other server returning our new `pulled_event`
@@ -305,21 +301,3 @@ class FederationClientTest(FederatingHomeserverTestCase):
         # This is 2 because it failed once from `self.OTHER_SERVER_NAME` and the
         # other from "yet.another.server"
         self.assertEqual(backfill_num_attempts, 2)
-
-
-def _mock_response(resp: JsonDict):
-    body = json.dumps(resp).encode("utf-8")
-
-    def deliver_body(p: Protocol):
-        p.dataReceived(body)
-        p.connectionLost(Failure(twisted.web.client.ResponseDone()))
-
-    response = mock.Mock(
-        code=200,
-        phrase=b"OK",
-        headers=twisted.web.client.Headers({"content-Type": ["application/json"]}),
-        length=len(body),
-        deliverBody=deliver_body,
-    )
-    mock.seal(response)
-    return response