Adds code coverage for FormatStringDialog

Related dotnet#10773

## Proposed changes

- Adds code coverage for `FormatStringDialog`.

## Customer Impact

- None

## Regression?

- No

## Risk

- Minimal

## Test methodology

- Unit tests

## Test environment(s)

- 9.0.100-rc.1.24452.12

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

@@ -0,0 +1,107 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+
+#nullable enable
+
+using System.ComponentModel;
+using Moq;
+
+namespace System.Windows.Forms.Design.Tests;
+
+public class FormatStringDialogTests : IDisposable
+{
+    private readonly FormatStringDialog _formatStringDialog;
+
+    private readonly Mock<ITypeDescriptorContext> _context = new(MockBehavior.Strict);
+
+    public FormatStringDialogTests() => _formatStringDialog = new(_context.Object);
+
+    public void Dispose() => _formatStringDialog.Dispose();
+
+    [Fact]
+    public void Ctor_WithContext_DoesNotThrow()
+    {
+        Action action = () => new FormatStringDialog(_context.Object);
+
+        action.Should().NotThrow();
+    }
+
+    [Fact]
+    public void FormatStringDialog_HasProperRTLConfiguration()
+    {
+        if (SR.RTL == "RTL_False")
+        {
+            _formatStringDialog!.RightToLeft.Should().Be(RightToLeft.No);
+            _formatStringDialog!.RightToLeftLayout.Should().Be(false);
+        }
+        else
+        {
+            _formatStringDialog!.RightToLeft.Should().Be(RightToLeft.Yes);
+            _formatStringDialog!.RightToLeftLayout.Should().Be(true);
+        }
+    }
+
+    [Fact]
+    public void SetsDataGridViewCellStyle_DoesNotThrow()
+    {
+        Action action = () => _formatStringDialog.DataGridViewCellStyle = new();
+
+        action.Should().NotThrow();
+    }
+
+    [Fact]
+    public void Dirty_ReturnsDefault() => _formatStringDialog.Dirty.Should().Be(false);
+
+    [Fact]
+    public void SetsListControl_DoesNotThrow()
+    {
+        Action action = () =>
+        {
+            using ListBox listControl = new();
+            _formatStringDialog.ListControl = listControl;
+        };
+        action.Should().NotThrow();
+    }
+
+    [Fact]
+    public void End_DoesNotThrow()
+    {
+        Action action = FormatStringDialog.End;
+
+        action.Should().NotThrow();
+    }
+
+    [Fact]
+    public void FormatControlFinishedLoading_AdjustsButtonPositionsCorrectly()
+    {
+        using dynamic? okButtonField = _formatStringDialog.TestAccessor().Dynamic._okButton;
+        using dynamic? cancelButtonField = _formatStringDialog.TestAccessor().Dynamic._cancelButton;
+        using dynamic? formatControlField = _formatStringDialog.TestAccessor().Dynamic._formatControl1;
+
+        int okButtonLeftOriginalState = okButtonField.Left;
+        int cancelButtonLeftOriginalState = cancelButtonField.Left;
+
+        int GetRightSideOffset(Control ctl)
+        {
+            int result = ctl.Width;
+            Control? control = ctl;
+            while (control is not null)
+            {
+                result += control.Left;
+                control = control.Parent;
+            }
+
+            return result;
+        }
+
+        int formatControlRightSideOffset = GetRightSideOffset(formatControlField);
+        int cancelButtonRightSideOffset = GetRightSideOffset(cancelButtonField);
+
+        _formatStringDialog.FormatControlFinishedLoading();
+
+        ((object)okButtonField.Top).Should().Be(formatControlField.Bottom + 5);
+        ((object)cancelButtonField.Top).Should().Be(formatControlField.Bottom + 5);
+        ((object)okButtonField.Left).Should().Be(okButtonLeftOriginalState + formatControlRightSideOffset - cancelButtonRightSideOffset);
+        ((object)cancelButtonField.Left).Should().Be(cancelButtonLeftOriginalState + formatControlRightSideOffset - cancelButtonRightSideOffset);
+    }
+}