Test running the macOS engine has no stray logging (#54716)

Assert that running the macOS engine does not log anything unexpected 
See also issue flutter/flutter#111577
Similar Windows check in #47774

[C++, Objective-C, Java style guides]: https://github.com/flutter/engine/blob/main/CONTRIBUTING.md#style

Author: jmagman

shell/platform/darwin/macos/framework/Source/FlutterEngineTest.mm

@@ -210,6 +210,7 @@ @implementation MockableFlutterEngine
 
   // Replace stdout stream buffer with our own.
   StreamCapture stdout_capture(&std::cout);
+  StreamCapture stderr_capture(&std::cerr);
 
   // Launch the test entrypoint.
   FlutterEngine* engine = GetFlutterEngine();
@@ -219,9 +220,12 @@ @implementation MockableFlutterEngine
   latch.Wait();
 
   stdout_capture.Stop();
+  stderr_capture.Stop();
 
   // Verify hello world was written to stdout.
-  EXPECT_TRUE(stdout_capture.GetOutput().find("Hello logging") != std::string::npos);
+  // Check equality to ensure no unexpected stray logging.
+  EXPECT_EQ(stdout_capture.GetOutput(), "flutter: Hello logging\n");
+  EXPECT_TRUE(stderr_capture.GetOutput().empty());
 }
 
 TEST_F(FlutterEngineTest, DISABLED_BackgroundIsBlack) {