Changes access of method

Related #10773

## Proposed changes

- Adds code coverage to `FormatStringEditor`

## 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/FormatStringEditorTests.cs

@@ -20,7 +20,7 @@ public class FormatStringEditorTests
  private Mock<ITypeDescriptorContext>? _context;
  private DataGridViewCellStyle? _cellStyle;
 
- public void ArrangeDependencies()
+ private void ArrangeDependencies()
  {
  _mockEditorService = new();
  _mockChangeService = new();