Add code coverage for CheckBox (#13455)

* Add code coverage for CheckBox

* Address FeedBacks

* Address FeedBacks

Author: Zheng-Li01

src/test/unit/System.Windows.Forms/System/Windows/Forms/CheckBoxTests.cs

@@ -873,5 +873,57 @@ public void CheckBox_ProcessMnemonic_ValidCases(bool useMnemonic, char charCode,
         checkBox.CheckState.Should().Be(CheckState.Checked);
     }
 
+    [WinFormsTheory]
+    [InlineData(Appearance.Button, FlatStyle.Standard,  "Test", 12, 8, 100, 20)]
+    [InlineData(Appearance.Normal, FlatStyle.System,    "Test", 12, 8, 100, 20)]
+    [InlineData(Appearance.Normal, FlatStyle.Flat,      "Test", 12, 8, 100, 20)]
+    [InlineData(Appearance.Normal, FlatStyle.Standard,  "Test", 12, 8, 100, 20)]
+    public void CheckBox_GetPreferredSizeCore_VariousStyles_ReturnsExpected(
+        Appearance appearance, FlatStyle flatStyle, string text, int fontSize, int padding, int width, int height)
+    {
+        using SubCheckBox control = (SubCheckBox)CreateButton();
+        control.Appearance = appearance;
+        control.FlatStyle = flatStyle;
+        control.Text = text;
+        control.Font = new Font(FontFamily.GenericSansSerif, fontSize);
+        control.Padding = new Padding(padding);
+
+        Size proposed = new(width, height);
+
+        if (appearance == Appearance.Button)
+        {
+            Size size = control.GetPreferredSizeCore(proposed);
+            size.Width.Should().BeGreaterThan(0);
+            size.Height.Should().BeGreaterThan(0);
+        }
+        else if (flatStyle == FlatStyle.System)
+        {
+            Size size = control.GetPreferredSizeCore(proposed);
+            size.Width.Should().BeGreaterThan(0);
+            size.Height.Should().BeGreaterThan(0);
+        }
+        else
+        {
+            Size size = control.GetPreferredSizeCore(proposed);
+            size.Width.Should().BeGreaterThan(0);
+            size.Height.Should().BeGreaterThan(0);
+        }
+    }
+
+    [WinFormsFact]
+    public void CheckBox_GetPreferredSizeCore_EnsuresMinimumHeight()
+    {
+        using SubCheckBox control = (SubCheckBox)CreateButton();
+        control.FlatStyle = FlatStyle.System;
+        control.Text = "A";
+        control.Font = new Font(FontFamily.GenericSansSerif, 1);
+        control.Padding = Padding.Empty;
+
+        Size proposed = new(1, 1);
+        Size result = control.GetPreferredSizeCore(proposed);
+
+        result.Height.Should().BeGreaterThanOrEqualTo(13);
+    }
+
     protected override ButtonBase CreateButton() => new SubCheckBox();
 }