tests/sanic: use a syntax for setting cookies that work on older versions

Author: xrmx

tests/contrib/sanic/fixtures.py

@@ -158,7 +158,7 @@ async def custom_headers(request):
         @app.get("/add-cookies")
         async def add_cookies(request):
             response = json({"data": "message"}, headers={"sessionid": 1234555})
-            response.add_cookie("some", "cookie")
+            response.cookies["some"] = "cookie"
             return response
 
         try: