[web] add comment to skipped safari test (flutter#16737)

* add comment to skipped safari test

* test not running semantics on Mac

* disable semantics test which is unreliable

Author: Nurhan Turgut

lib/web_ui/test/engine/history_test.dart

@@ -4,6 +4,7 @@
 
 // @dart = 2.6
 @TestOn('vm && linux')
+// TODO(nurhan): https://github.com/flutter/flutter/issues/51169
 
 import 'dart:async';
 import 'dart:typed_data';

lib/web_ui/test/engine/semantics/semantics_test.dart

@@ -3,6 +3,9 @@
 // found in the LICENSE file.
 
 // @dart = 2.6
+@TestOn('vm && linux')
+// TODO(nurhan): https://github.com/flutter/flutter/issues/50590
+
 import 'dart:async';
 import 'dart:html' as html;
 import 'dart:typed_data';