Adds code coverage to FormatControl (#12168)

Related #10773

## Proposed changes

- Adds code coverage to `FormatControl`.

## Customer Impact

- None

## Regression?

- No

## Risk

- Minimal

## Test methodology

- Unit tests

## Test environment(s)

- 9.0.100-rc.1.24452.12

Co-authored-by: Ricardo Bossan (BEYONDSOFT CONSULTING INC) (from Dev Box) <[email protected]>

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

@@ -0,0 +1,44 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+
+#nullable enable
+
+namespace System.Windows.Forms.Design.Tests;
+public class FormatControlTests : IDisposable
+{
+    private readonly FormatControl _formatControl = new();
+    public void Dispose() => _formatControl.Dispose();
+
+    [Fact]
+    public void DefaultProperties_ShouldReturnExpected()
+    {
+        _formatControl.Should().NotBeNull();
+        _formatControl.Should().BeOfType<FormatControl>();
+        _formatControl.Should().BeAssignableTo<Control>();
+        _formatControl.Dirty.Should().BeFalse();
+        _formatControl.FormatType.Should().BeEmpty();
+        _formatControl.NullValue.Should().BeNull();
+    }
+
+    [Fact]
+    public void SetsNullValueTextBoxEnabled_ShouldNotThrow()
+    {
+        _formatControl.Invoking(f => f.NullValueTextBoxEnabled = false).Should().NotThrow();
+    }
+
+    public static IEnumerable<object[]> TestData()
+    {
+        yield return new object[] { null!, SR.BindingFormattingDialogFormatTypeNoFormatting };
+        yield return new object[] { "N1", SR.BindingFormattingDialogFormatTypeNumeric };
+        yield return new object[] { "d", SR.BindingFormattingDialogFormatTypeDateTime };
+        yield return new object[] { "E1", SR.BindingFormattingDialogFormatTypeScientific };
+        yield return new object[] { "CustomString", SR.BindingFormattingDialogFormatTypeCustom };
+    }
+
+    [Theory]
+    [MemberData(nameof(TestData))]
+    public void FormatTypeStringFromFormatString_ShouldReturnCorrectFormatType(string formatString, string formatTypeString) => FormatControl.FormatTypeStringFromFormatString(formatString).Should().Be(formatTypeString);
+
+    [Fact]
+    public void ResetFormattingInfo_ShouldNotThrow() => _formatControl.Invoking(f => f.ResetFormattingInfo()).Should().NotThrow();
+}