Disable super flakey impeller test. (#144573)

Until we figure out why this is unstable on Impeller swiftshader, disable it.

packages/flutter/test/widgets/shader_mask_test.dart

@@ -10,6 +10,8 @@ library;
 import 'package:flutter/widgets.dart';
 import 'package:flutter_test/flutter_test.dart';
 
+import '../impeller_test_helpers.dart';
+
 Shader createShader(Rect bounds) {
   return const LinearGradient(
     begin: Alignment.topCenter,
@@ -108,5 +110,5 @@ void main() {
       find.byType(RepaintBoundary),
       matchesGoldenFile('shader_mask.bounds.matches_top_left.png'),
     );
-  });
+  }, skip: impellerEnabled); // https://github.com/flutter/flutter/issues/144555
 }