add test for tooltip

tests/Avalonia.Controls.UnitTests/ToolTipTests.cs

@@ -0,0 +1,45 @@
+// Copyright (c) The Avalonia Project. All rights reserved.
+// Licensed under the MIT license. See licence.md file in the project root for full license information.
+
+using System;
+using System.Reactive.Disposables;
+using Avalonia.Platform;
+using Avalonia.Threading;
+using Avalonia.UnitTests;
+using Avalonia.VisualTree;
+using Moq;
+using Xunit;
+
+namespace Avalonia.Controls.UnitTests
+{
+    public class TolTipTests
+    {
+        private MouseTestHelper _mouseHelper = new MouseTestHelper();
+
+        [Fact]
+        public void Should_Open_On_Pointer_Enter()
+        {
+            using (UnitTestApplication.Start(TestServices.StyledWindow))
+            {
+                var window = new Window();
+
+                var target = new Decorator()
+                {
+                    [ToolTip.TipProperty] = "Tip",
+                    [ToolTip.ShowDelayProperty] = 0
+                };
+
+                window.Content = target;
+
+                window.ApplyTemplate();
+                window.Presenter.ApplyTemplate();
+
+                Assert.True((target as IVisual).IsAttachedToVisualTree);
+
+                _mouseHelper.Enter(target);
+
+                Assert.True(ToolTip.GetIsOpen(target));
+            }
+        }
+    }
+}