Move unit tests for shared components to System.Private.Windows.Core.Tests

[Tanya-Solyanik]

Jeremy is currently moving Clipboard and drag and drop-related implementation code to System.Private.Windows.Core assembly. This assembly does not reference System.Windows.Forms project because it is lower in the layering.
Some classes that were moved, such as https://github.com/dotnet/winforms/blob/main/src/System.Private.Windows.Core/src/System/Private/Windows/Ole/BinaryFormatUtilities.cs
have dedicated tests in System.Windows.Forms.Tests assembly. These tests can be moved to a new test assembly (SystemPrivate.Windows.Core) that does not use System.Windows.Forms namespace. Tests would require Mocks for the new interfaces to avoid taking dependency on the System.Windows.Forms project.
Some test utilities can be moved from the System.Windows.Forms.Tests into System.Private.Windows.Core.TestUtilities, for example -

winforms/src/System.Windows.Forms/tests/UnitTests/System/Windows/Forms/BinaryFormatUtilitiesTests.BinaryFormatterFullCompatScope.cs

Line 6 in 23cb171

public partial class BinaryFormatUtilitiesTests