Update topenAIChat.m

Changing createOpenAIChatWithStreamFunc as suggested by Christopher

tests/topenAIChat.m

@@ -126,10 +126,23 @@ function noStopSequencesNoMaxNumTokens(testCase)
         end
 
         function createOpenAIChatWithStreamFunc(testCase)
-            sf = @(x)fprintf("%s", x);
-            chat = openAIChat(ApiKey="this-is-not-a-real-key", StreamFun=sf);
+
+            function seen = sf(str)
+                persistent data;
+                if isempty(data)
+                    data = strings(1, 0);
+                end
+                % Append streamed text to an empty string array of length 1
+                data = [data, str];
+                seen = data;
+            end
+            chat = openAIChat(ApiKey=getenv("OPENAI_KEY"), StreamFun=@sf);
 
             testCase.verifyWarningFree(@()generate(chat, "Hello world."));
+            % Checking that persistent data, which is still stored in
+            % memory, is greater than 1. This would mean that the stream
+            % function has been called and streamed some text.
+            testCase.verifyGreaterThan(numel(sf("")), 1);
         end
 
         function warningJSONResponseFormatGPT35(testCase)