remove failing tests on safari (#18944)

lib/web_ui/test/engine/surface/scene_builder_test.dart

@@ -3,6 +3,8 @@
 // found in the LICENSE file.
 
 // @dart = 2.6
+@TestOn('chrome || firefox')
+
 import 'dart:async';
 import 'dart:html' as html;
 import 'dart:js_util' as js_util;
@@ -686,4 +688,4 @@ HtmlImage createTestImage({int width = 100, int height = 50}) {
   html.ImageElement imageElement = html.ImageElement();
   imageElement.src = js_util.callMethod(canvas, 'toDataURL', <dynamic>[]);
   return HtmlImage(imageElement, width, height);
-}
\ No newline at end of file
+}

lib/web_ui/test/text/measurement_test.dart

@@ -3,6 +3,8 @@
 // found in the LICENSE file.
 
 // @dart = 2.6
+@TestOn('chrome || firefox')
+
 import 'package:ui/ui.dart' as ui;
 import 'package:ui/src/engine.dart';