fix: Call codec.dispose in flutter/test (#161127)

PR derived from https://github.com/flutter/flutter/pull/159945.

Added `codec.dispose()` for directories under `flutter/test`.

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packages/flutter/test/painting/fake_codec.dart

@@ -30,7 +30,9 @@ class FakeCodec implements ui.Codec {
     for (int i = 0; i < frameCount; i += 1) {
       frameInfos.add(await codec.getNextFrame());
     }
-    return FakeCodec._(frameCount, codec.repetitionCount, frameInfos);
+    final int repetitionCount = codec.repetitionCount;
+    codec.dispose();
+    return FakeCodec._(frameCount, repetitionCount, frameInfos);
   }
 
   @override

packages/flutter/test/widgets/image_filter_quality_test.dart

@@ -126,6 +126,7 @@ Future<void> testImageQuality(WidgetTester tester, ui.FilterQuality? quality) as
   final ui.Image image =
       (await tester.runAsync(() async {
         final ui.Codec codec = await ui.instantiateImageCodec(test3x3Image);
+        addTearDown(codec.dispose);
         return (await codec.getNextFrame()).image;
       }))!;
   addTearDown(image.dispose);

packages/flutter/test/widgets/widget_inspector_test.dart

@@ -4717,6 +4717,7 @@ class _TestWidgetInspectorService extends TestWidgetInspectorService {
                 base64.decode(base64Screenshot),
               );
               final ui.FrameInfo frame = await codec.getNextFrame();
+              codec.dispose();
               return frame.image;
             }))!;
         addTearDown(screenshotImage.dispose);