Made tests for the AutoCompleteMode and TextSelector properties

tests/Avalonia.Controls.UnitTests/AutoCompleteBoxTests.cs

@@ -363,6 +363,47 @@ namespace Avalonia.Controls.UnitTests
             });
         }
         
+        [Fact]
+        public void Test_Selectors()
+        {
+            Assert.Equal(GetSelector(AutoCompleteMode.Replace)("Never", "gonna"), "gonna");
+            Assert.Equal(GetSelector(AutoCompleteMode.Replace)("give", "you"), "you");
+            Assert.NotEqual(GetSelector(AutoCompleteMode.Replace)("up", "!"), "42");
+        }
+
+        [Fact]
+        public void AutoCompleteMode_Changes_To_Custom_And_Back()
+        {
+            RunTest((control, textbox) =>
+            {
+                Assert.Equal(control.AutoCompleteMode, AutoCompleteMode.Replace);
+
+                control.TextSelector = (text, item) => text + item;
+                Assert.Equal(control.AutoCompleteMode, AutoCompleteMode.Custom);
+
+                control.AutoCompleteMode = AutoCompleteMode.Replace;
+                Assert.Equal(control.AutoCompleteMode, AutoCompleteMode.Replace);
+                Assert.Equal(control.TextSelector, GetSelector(AutoCompleteMode.Replace));
+            });
+        }
+
+        [Fact]
+        public void Custom_TextSelector()
+        {
+            RunTest((control, textbox) =>
+            {
+                object selectedItem = control.Items.Cast<object>().First();
+                string input = "42";
+
+                control.TextSelector = (text, item) => text + item;
+                Assert.Equal(control.TextSelector("4", "2"), "42");
+
+                control.Text = input;
+                control.SelectedItem = selectedItem;
+                Assert.Equal(control.Text, control.TextSelector(input, selectedItem.ToString()));
+            });
+        }
+
         /// <summary>
         /// Retrieves a defined predicate filter through a new AutoCompleteBox 
         /// control instance.
@@ -375,6 +416,17 @@ namespace Avalonia.Controls.UnitTests
                 .TextFilter;
         }
 
+        /// <summary>
+        /// Retrieves a defined selector through a new AutoCompleteBox 
+        /// control instance.
+        /// </summary>
+        /// <param name="mode">The AutoCompleteMode of interest.</param>
+        /// <returns>Returns the selector instance.</returns>
+        private static AutoCompleteSelector<string> GetSelector(AutoCompleteMode mode)
+        {
+            return new AutoCompleteBox { AutoCompleteMode = mode }.TextSelector;
+        }
+
         /// <summary>
         /// Creates a large list of strings for AutoCompleteBox testing.
         /// </summary>