Add the number of mismatched pixels to golden tests output (#142975)

Fixes https://github.com/flutter/flutter/issues/141036

packages/flutter_test/lib/src/_goldens_io.dart

@@ -262,7 +262,7 @@ Future<ComparisonResult> compareLists(List<int>? test, List<int>? master) async
       passed: false,
       diffPercent: diffPercent,
       error: 'Pixel test failed, '
-        '${(diffPercent * 100).toStringAsFixed(2)}% '
+        '${(diffPercent * 100).toStringAsFixed(2)}%, ${pixelDiffCount}px '
         'diff detected.',
       diffs:  <String, Image>{
         'masterImage' : masterImage,

packages/flutter_test/test/goldens_test.dart

@@ -192,7 +192,7 @@ void main() {
             throwsA(isFlutterError.having(
               (FlutterError error) => error.message,
               'message',
-              contains('% diff detected'),
+              contains('100.00%, 1px diff detected'),
             )),
           );
           final io.File master = fs.file(
@@ -223,7 +223,7 @@ void main() {
             throwsA(isFlutterError.having(
               (FlutterError error) => error.message,
               'message',
-              contains('% diff detected'),
+              contains('100.00%, 1px diff detected'),
             )),
           );
           final io.File master = fs.file(
@@ -302,7 +302,7 @@ void main() {
             throwsA(isFlutterError.having(
               (FlutterError error) => error.message,
               'message',
-              contains('% diff detected'),
+              contains('100.00%, 1px diff detected'),
             )),
           );
         });