Need tests for SignCheck - part I (Post Build Signing changes)

[missymessa]

This is a legacy project, however, we have made some changes to it for the Post Build Signing epic so we should have tests added to cover these changes (per this PR https://github.com/dotnet/arcade/pull/5210/files).

• Create a unit test project for SignCheck in Arcade
• GetInputFilesFromOptions function in SignCheck.cs (as this is currently a private method, we may need to use Reflection or the PrivateObject functionality for testing)
• Unit tests for the entire SignCheckTask class

[chcosta]

Closing associated PR because Arcade doesn't have support required to run SignCheck UnitTests at the moment. I'll move this issue out of the post build signing epic and it can go to triage.