Added tests from nthmaster.com

tests/Avalonia.Styling.UnitTests/SelectorTests_NthChild.cs

@@ -1,3 +1,4 @@
+using System.Linq;
 using System.Threading.Tasks;
 using Avalonia.Controls;
 using Xunit;
@@ -205,6 +206,76 @@ namespace Avalonia.Styling.UnitTests
             Assert.Equal(SelectorMatch.NeverThisInstance, target.Match(b1));
         }
 
+
+        [Theory] // http://nthmaster.com/
+        [InlineData(+0, 8, false, false, false, false, false, false, false, true , false, false, false)]
+        [InlineData(+1, 6, false, false, false, false, false, true , true , true , true , true , true )]
+        [InlineData(-1, 9, true , true , true , true , true , true , true , true , true , false, false)]
+        public async Task Nth_Child_Master_Com_Test_Sigle_Selector(
+            int step, int offset, params bool[] items)
+        {
+            var panel = new StackPanel();
+            panel.Children.AddRange(items.Select(_ => new Border()));
+
+            var previous = default(Selector).OfType<Border>();
+            var target = previous.NthChild(step, offset);
+
+            var results = new bool[items.Length];
+            for (int index = 0; index < items.Length; index++)
+            {
+                var border = panel.Children[index];
+                results[index] = await target.Match(border).Activator!.Take(1);
+            }
+
+            Assert.Equal(items, results);
+        }
+
+        [Theory] // http://nthmaster.com/
+        [InlineData(+1, 4, -1, 8, false, false, false, true , true , true , true , true , false, false, false)]
+        [InlineData(+3, 1, +2, 0, false, false, false, true , false, false, false, false, false, true , false)]
+        public async Task Nth_Child_Master_Com_Test_Double_Selector(
+            int step1, int offset1, int step2, int offset2, params bool[] items)
+        {
+            var panel = new StackPanel();
+            panel.Children.AddRange(items.Select(_ => new Border()));
+
+            var previous = default(Selector).OfType<Border>();
+            var middle = previous.NthChild(step1, offset1);
+            var target = middle.NthChild(step2, offset2);
+
+            var results = new bool[items.Length];
+            for (int index = 0; index < items.Length; index++)
+            {
+                var border = panel.Children[index];
+                results[index] = await target.Match(border).Activator!.Take(1);
+            }
+
+            Assert.Equal(items, results);
+        }
+
+        [Theory] // http://nthmaster.com/
+        [InlineData(+1, 2, 2, 1, -1, 9, false, false, true , false, true , false, true , false, true , false, false)]
+        public async Task Nth_Child_Master_Com_Test_Triple_Selector(
+            int step1, int offset1, int step2, int offset2, int step3, int offset3, params bool[] items)
+        {
+            var panel = new StackPanel();
+            panel.Children.AddRange(items.Select(_ => new Border()));
+
+            var previous = default(Selector).OfType<Border>();
+            var middle1 = previous.NthChild(step1, offset1);
+            var middle2 = middle1.NthChild(step2, offset2);
+            var target = middle2.NthChild(step3, offset3);
+
+            var results = new bool[items.Length];
+            for (int index = 0; index < items.Length; index++)
+            {
+                var border = panel.Children[index];
+                results[index] = await target.Match(border).Activator!.Take(1);
+            }
+
+            Assert.Equal(items, results);
+        }
+
         [Fact]
         public void Returns_Correct_TargetType()
         {