fix: Call codec.dispose in flutter_test (#161131)

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/lib/src/_goldens_io.dart

@@ -188,10 +188,12 @@ Future<ComparisonResult> compareLists(List<int>? test, List<int>? master) async
 
   final Codec testImageCodec = await instantiateImageCodec(Uint8List.fromList(test));
   final Image testImage = (await testImageCodec.getNextFrame()).image;
+  testImageCodec.dispose();
   final ByteData? testImageRgba = await testImage.toByteData();
 
   final Codec masterImageCodec = await instantiateImageCodec(Uint8List.fromList(master));
   final Image masterImage = (await masterImageCodec.getNextFrame()).image;
+  masterImageCodec.dispose();
   final ByteData? masterImageRgba = await masterImage.toByteData();
 
   final int width = testImage.width;