test: WIP, fix an issue when passing year event.

tests/HeadlessTest.Ursa/Controls/DateTimePicker/CalendarDayButtonTests.cs

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+using Avalonia;
+using Avalonia.Controls;
+using Avalonia.Headless;
+using Avalonia.Headless.XUnit;
+using Avalonia.Input;
+using Avalonia.Threading;
+using Ursa.Controls;
+
+namespace HeadlessTest.Ursa.Controls.DateTimePicker;
+
+public class CalendarDayButtonTests
+{
+    [AvaloniaFact]
+    public void OnPointerReleased_RaisesDateSelectedEvent()
+    {
+        Window window = new Window();
+        var button = new CalendarDayButton();
+        var date = new DateTime(2023, 5, 15);
+        button.DataContext = date;
+        window.Content = button;
+        window.Show();
+        Dispatcher.UIThread.RunJobs();
+        int eventRaised = 0;
+        DateTime? eventContext = null;
+
+        void OnMouseClick(object? sender, CalendarDayButtonEventArgs args)
+        {
+            eventRaised++;
+            eventContext = args.Date;
+        }
+
+        button.DateSelected += OnMouseClick;
+        window.MouseUp(new Point(10, 10), MouseButton.Left);
+        Assert.Equal(1, eventRaised);
+        Assert.Equal(date, eventContext);
+        button.DateSelected -= OnMouseClick;
+        eventContext = null;
+        window.MouseUp(new Point(10, 10), MouseButton.Left);
+        Assert.Null(eventContext);
+        Assert.Equal(1, eventRaised);
+    }
+
+    [AvaloniaFact]
+    public void OnPointerEntered_RaisesDatePreviewedEvent()
+    {
+        Window window = new Window();
+        var button = new CalendarDayButton();
+        var date = new DateTime(2023, 5, 15);
+        button.DataContext = date;
+        window.Content = button;
+        window.Show();
+        Dispatcher.UIThread.RunJobs();
+        int eventRaised = 0;
+        DateTime? eventContext = null;
+
+        void OnMouseEnter(object? sender, CalendarDayButtonEventArgs args)
+        {
+            eventRaised++;
+            eventContext = args.Date;
+        }
+
+        button.DatePreviewed += OnMouseEnter;
+        window.MouseMove(new Point(10, 10));
+        Assert.Equal(1, eventRaised);
+        Assert.Equal(date, eventContext);
+        window.MouseMove(new Point(100, 100));
+        button.DatePreviewed -= OnMouseEnter;
+        eventContext = null;
+        window.MouseMove(new Point(10, 10));
+        Assert.Null(eventContext);
+        Assert.Equal(1, eventRaised);
+    }
+}