remove unnecessary artificial delay in catalog example (#31822)

* remove unnecessary artificial delay

* fix test_driver README

examples/catalog/bin/screenshot_test.dart.template

@@ -25,8 +25,6 @@ void main() {
       ];
       for (String path in paths) {
         await driver.waitUntilNoTransientCallbacks();
-        // TBD: when #11021 has been resolved, this shouldn't be necessary.
-        await new Future<void>.delayed(const Duration(milliseconds: 500));
         final List<int> pixels = await driver.screenshot();
         final File file = new File(path);
         await file.writeAsBytes(pixels);

examples/catalog/test_driver/README.md

-The screenshot_test.dart and screenshot_test.dart files were generated by ../bin/sample_page.dart. They should not be checked in.
+The screenshot.dart and screenshot_test.dart files were generated by ../bin/sample_page.dart. They should not be checked in.