Preparatory refactoring of the SamlHandlerTestCase (#8938)

* move simple_async_mock to test_utils

... so that it can be re-used

* Remove references to `SamlHandler._map_saml_response_to_user` from tests

This method is going away, so we can no longer use it as a test point. Instead,
factor out a higher-level method which takes a SAML object, and verify correct
behaviour by mocking out `AuthHandler.complete_sso_login`.

* changelog

tests/test_utils/__init__.py

@@ -22,6 +22,8 @@ import warnings
 from asyncio import Future
 from typing import Any, Awaitable, Callable, TypeVar
 
+from mock import Mock
+
 import attr
 
 from twisted.python.failure import Failure
@@ -87,6 +89,16 @@ def setup_awaitable_errors() -> Callable[[], None]:
     return cleanup
 
 
+def simple_async_mock(return_value=None, raises=None) -> Mock:
+    # AsyncMock is not available in python3.5, this mimics part of its behaviour
+    async def cb(*args, **kwargs):
+        if raises:
+            raise raises
+        return return_value
+
+    return Mock(side_effect=cb)
+
+
 @attr.s
 class FakeResponse:
     """A fake twisted.web.IResponse object