transaction_tests: Remove testing URL.

Rewrite the test so that we don't have a dedicated URL for testing.
dev_update_subgroups is called directly from the tests without using the
test client.

zproject/dev_urls.py

@@ -10,7 +10,6 @@ from django.urls import path
 from django.views.generic import TemplateView
 from django.views.static import serve
 
-from zerver.lib.rest import rest_path
 from zerver.views.auth import config_error, login_page
 from zerver.views.development.cache import remove_caches
 from zerver.views.development.camo import handle_camo_url
@@ -32,7 +31,6 @@ from zerver.views.development.registration import (
     register_development_realm,
     register_development_user,
 )
-from zerver.views.development.user_groups import dev_update_subgroups
 
 # These URLs are available only in the development environment
 
@@ -100,14 +98,6 @@ urls = [
     path("external_content/<digest>/<received_url>", handle_camo_url),
 ]
 
-testing_urls = [
-    rest_path(
-        "testing/user_groups/<int:user_group_id>/subgroups",
-        POST=(dev_update_subgroups, {"intentionally_undocumented"}),
-    ),
-]
-urls += testing_urls
-
 v1_api_mobile_patterns = [
     # This is for the signing in through the devAuthBackEnd on mobile apps.
     path("dev_fetch_api_key", api_dev_fetch_api_key),