Add app.test_mode which is set on testing calls

sanic/app.py

@@ -90,6 +90,7 @@ class Sanic:
         self.named_response_middleware = {}
         # Register alternative method names
         self.go_fast = self.run
+        self.test_mode = False
 
     @property
     def loop(self):

sanic/testing.py

@@ -21,6 +21,7 @@ class SanicTestClient:
         self.app = app
         self.port = port
         self.host = host
+        self.app.test_mode = True
 
     def get_new_session(self):
         return httpx.AsyncClient(verify=False)
@@ -200,6 +201,7 @@ class SanicASGITestClient(httpx.AsyncClient):
         app.asgi = True
 
         self.app = app
+        self.app.test_mode = True
 
         dispatch = SanicASGIDispatch(app=app, client=(ASGI_HOST, PORT or 0))
         super().__init__(dispatch=dispatch, base_url=base_url)