add test, thanks @donandren ! :D

tests/Avalonia.Visuals.UnitTests/Media/PathSegmentTests.cs

@@ -0,0 +1,34 @@
+using Avalonia.Media;
+using Xunit;
+
+namespace Avalonia.Visuals.UnitTests.Media
+{
+    public class PathSegmentTests
+    {
+        [Fact]
+        public void PathSegment_Triggers_Invalidation_On_Property_Change()
+        {
+            var targetSegment = new ArcSegment()
+            {
+                Size = new Size(10, 10),
+                Point = new Point(5, 5)
+            };
+
+            var target = new PathGeometry
+            {
+                Figures = new PathFigures
+                {
+                    new PathFigure { IsClosed = false, Segments = new PathSegments { targetSegment } }
+                }
+            };
+            
+            var changed = false;
+
+            target.Changed += (s, e) => changed = true;
+
+            targetSegment.Size = new Size(20, 20);
+
+            Assert.True(changed);
+        }
+    }
+}