tests: Refactor `social_auth_test`.

As "choose email" screen is only used for GitHub auth, the part
that deals with it is separated from `social_auth_test` and
dealt in a new function `social_auth_finish`. This new
`social_auth_finish` contains only the code that deals with
authentication backends that do not have "choose email" screen.
But it is overidden in GitHub test class to handle the
"choose email" screen.
It was refactored because `expect_choose_email_screen` blocks
were confusing while figuring out how tests work on non GitHub
auths.

zerver/tests/test_auth_backends.py

@@ -707,6 +707,16 @@ class SocialAuthBase(DesktopFlowTestingLib, ZulipTestCase):
 
         return url, headers
 
+    def social_auth_test_finish(self, result: HttpResponse,
+                                account_data_dict: Dict[str, str],
+                                expect_choose_email_screen: bool,
+                                **headers: Any) -> HttpResponse:
+        parsed_url = urllib.parse.urlparse(result.url)
+        csrf_state = urllib.parse.parse_qs(parsed_url.query)['state']
+        result = self.client_get(self.AUTH_FINISH_URL,
+                                 dict(state=csrf_state), **headers)
+        return result
+
     def social_auth_test(self, account_data_dict: Dict[str, str],
                          *, subdomain: Optional[str]=None,
                          mobile_flow_otp: Optional[str]=None,
@@ -765,34 +775,9 @@ class SocialAuthBase(DesktopFlowTestingLib, ZulipTestCase):
             )
             self.register_extra_endpoints(requests_mock, account_data_dict, **extra_data)
 
-            parsed_url = urllib.parse.urlparse(result.url)
-            csrf_state = urllib.parse.parse_qs(parsed_url.query)['state']
-            result = self.client_get(self.AUTH_FINISH_URL,
-                                     dict(state=csrf_state), **headers)
-
-            if expect_choose_email_screen and result.status_code == 200:
-                # For authentication backends such as GitHub that
-                # successfully authenticate multiple email addresses,
-                # we'll have an additional screen where the user selects
-                # which email address to login using (this screen is a
-                # "partial" state of the python-social-auth pipeline).
-                #
-                # TODO: Generalize this testing code for use with other
-                # authentication backends; for now, we just assert that
-                # it's definitely the GitHub authentication backend.
-                if self.AUTH_FINISH_URL == "/complete/github/":
-                    self.assert_in_success_response(["Select account"], result)
-                    result = self.client_get(self.AUTH_FINISH_URL,
-                                             dict(state=csrf_state, email=account_data_dict['email']), **headers)
-            elif self.AUTH_FINISH_URL == "/complete/github/":
-                # We want to be explicit about when we expect a test to
-                # use the "choose email" screen, but of course we should
-                # only check for that screen with the GitHub backend,
-                # because this test code is shared with other
-                # authentication backends that structurally will never use
-                # that screen.
-                assert not expect_choose_email_screen
-
+            result = self.social_auth_test_finish(result, account_data_dict,
+                                                  expect_choose_email_screen,
+                                                  **headers)
         return result
 
     def test_social_auth_no_key(self) -> None:
@@ -1690,6 +1675,39 @@ class GitHubAuthBackendTest(SocialAuthBase):
     AUTH_FINISH_URL = "/complete/github/"
     CONFIG_ERROR_URL = "/config-error/github"
 
+    def social_auth_test_finish(self, result: HttpResponse,
+                                account_data_dict: Dict[str, str],
+                                expect_choose_email_screen: bool,
+                                **headers: Any) -> HttpResponse:
+        parsed_url = urllib.parse.urlparse(result.url)
+        csrf_state = urllib.parse.parse_qs(parsed_url.query)['state']
+        result = self.client_get(self.AUTH_FINISH_URL,
+                                 dict(state=csrf_state), **headers)
+
+        if expect_choose_email_screen and result.status_code == 200:
+            # As GitHub authenticates multiple email addresses,
+            # we'll have an additional screen where the user selects
+            # which email address to login using (this screen is a
+            # "partial" state of the python-social-auth pipeline).
+            #
+            # TODO: Generalize this testing code for use with other
+            # authentication backends when a new authentacation backend
+            # that requires "choose email" screen; for now, we just assert
+            # that it's definitely the GitHub authentication backend.
+            if self.AUTH_FINISH_URL == "/complete/github/":
+                self.assert_in_success_response(["Select account"], result)
+                result = self.client_get(self.AUTH_FINISH_URL,
+                                         dict(state=csrf_state, email=account_data_dict['email']), **headers)
+        elif self.AUTH_FINISH_URL == "/complete/github/":
+            # We want to be explicit about when we expect a test to
+            # use the "choose email" screen, but of course we should
+            # only check for that screen with the GitHub backend,
+            # because this test code is shared with other
+            # authentication backends that structurally will never use
+            # that screen.
+            assert not expect_choose_email_screen
+        return result
+
     def register_extra_endpoints(self, requests_mock: responses.RequestsMock,
                                  account_data_dict: Dict[str, str],
                                  **extra_data: Any) -> None: