Add basic unit tests for empty DockPanel with LastChildFill = true.

Validated repro for fix given in #618

Author: Arlodotexe

components/Primitives/tests/DockPanel/DockPanelSample.xaml

@@ -0,0 +1,18 @@
+<Page x:Class="PrimitivesTests.DockPanelSample"
+      xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
+      xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
+      xmlns:controls="using:CommunityToolkit.WinUI.Controls"
+      xmlns:converters="using:CommunityToolkit.WinUI.Converters"
+      xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
+      xmlns:local="using:PrimitivesTests"
+      xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
+      xmlns:ui="using:CommunityToolkit.WinUI"
+      mc:Ignorable="d">
+
+    <controls:DockPanel x:Name="SampleDockPanel"
+                            Grid.Column="1"
+                            HorizontalSpacing="5"
+                            LastChildFill="True"
+                            VerticalSpacing="5">
+    </controls:DockPanel>
+</Page>

components/Primitives/tests/DockPanel/DockPanelSample.xaml.cs

@@ -0,0 +1,14 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+// See the LICENSE file in the project root for more information.
+
+namespace PrimitivesTests;
+
+public sealed partial class DockPanelSample : Page
+{
+    public DockPanelSample()
+    {
+        this.InitializeComponent();
+    }
+}
+

components/Primitives/tests/DockPanel/DockPanelTests.cs

@@ -0,0 +1,16 @@
+using CommunityToolkit.Tests;
+using CommunityToolkit.Tooling.TestGen;
+
+namespace PrimitivesTests;
+
+[TestClass]
+public partial class DockPanelTests : VisualUITestBase
+{
+    [UIThreadTestMethod]
+    public void DockPanelTest(DockPanelSample page)
+    {
+        var dockPanel = page.FindDescendant<CommunityToolkit.WinUI.Controls.DockPanel>();
+
+        Assert.IsNotNull(dockPanel, "Couldn't find DockPanel");
+    }
+}

components/Primitives/tests/Primitives.Tests.projitems

@@ -9,6 +9,10 @@
     <Import_RootNamespace>PrimitivesTests</Import_RootNamespace>
   </PropertyGroup>
   <ItemGroup>
+    <Compile Include="$(MSBuildThisFileDirectory)DockPanel\DockPanelSample.xaml.cs">
+      <DependentUpon>DockPanelSample.xaml</DependentUpon>
+    </Compile>
+    <Compile Include="$(MSBuildThisFileDirectory)DockPanel\DockPanelTests.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)SwitchPresenter\SwitchConverterBrushSample.xaml.cs">
       <DependentUpon>%(Filename)</DependentUpon>
     </Compile>
@@ -35,6 +39,10 @@
     </Compile>
   </ItemGroup>
   <ItemGroup>
+    <Page Include="$(MSBuildThisFileDirectory)DockPanel\DockPanelSample.xaml">
+      <SubType>Designer</SubType>
+      <Generator>MSBuild:Compile</Generator>
+    </Page>
     <Page Include="$(MSBuildThisFileDirectory)UniformGrid\AutoLayoutFixedElementZeroZeroSpecialPage.xaml">
       <SubType>Designer</SubType>
       <Generator>MSBuild:Compile</Generator>