Add test for design binding picker (#13536)

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Related #10773

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Author: LeafShi1

src/System.Windows.Forms.Design/tests/UnitTests/System/Windows/Forms/Design/DesignBindingPickerTests.cs

@@ -0,0 +1,71 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+
+using System.Drawing;
+using static System.Windows.Forms.Design.DesignBindingPicker;
+
+namespace System.Windows.Forms.Design.Tests;
+
+public class DesignBindingPickerTests : IDisposable
+{
+    private readonly DesignBindingPicker _picker;
+    private readonly BindingPickerTree _treeViewCtrl;
+    private readonly BindingPickerLink _addNewCtrl;
+    private readonly Panel _addNewPanel;
+    private readonly HelpTextLabel _helpTextCtrl;
+    private readonly Panel _helpTextPanel;
+
+    public DesignBindingPickerTests()
+    {
+        _picker = new DesignBindingPicker();
+        _treeViewCtrl = _picker.TestAccessor().Dynamic._treeViewCtrl;
+        _addNewCtrl = _picker.TestAccessor().Dynamic._addNewCtrl;
+        _addNewPanel = _picker.TestAccessor().Dynamic._addNewPanel;
+        _helpTextCtrl = _picker.TestAccessor().Dynamic._helpTextCtrl;
+        _helpTextPanel = _picker.TestAccessor().Dynamic._helpTextPanel;
+    }
+
+    public void Dispose() => _picker.Dispose();
+
+    [WinFormsFact]
+    public void Ctor_InitializesControlsAndProperties()
+    {
+        // Check control hierarchy.
+        var controls = _picker.Controls;
+
+        controls.Contains(_treeViewCtrl).Should().BeTrue();
+        controls.Contains(_addNewPanel).Should().BeTrue();
+        controls.Contains(_helpTextPanel).Should().BeTrue();
+
+        // Check some property values.
+        Assert.Equal(SystemColors.Control, _picker.BackColor);
+        Assert.Equal(SR.DesignBindingPickerAccessibleName, _picker.AccessibleName);
+        Assert.Equal(_treeViewCtrl, _picker.ActiveControl);
+    }
+
+    [WinFormsFact]
+    public void AddNewPanel_ContainsExpectedControls()
+    {
+        _addNewPanel.Controls.Contains(_addNewCtrl).Should().BeTrue();
+        ContainsType<PictureBox>(_addNewPanel.Controls).Should().BeTrue();
+        ContainsType<Label>(_addNewPanel.Controls).Should().BeTrue();
+    }
+
+    [WinFormsFact]
+    public void HelpTextPanel_ContainsExpectedControls()
+    {
+        _helpTextPanel.Controls.Contains(_helpTextCtrl).Should().BeTrue();
+        ContainsType<Label>(_helpTextPanel.Controls).Should().BeTrue();
+    }
+
+    private static bool ContainsType<T>(Control.ControlCollection collection)
+    {
+        foreach (var item in collection)
+        {
+            if (item is T)
+                return true;
+        }
+
+        return false;
+    }
+}