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

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).

- [x] Create a unit test project for SignCheck in Arcade
- [x] GetInputFilesFromOptions function in SignCheck.cs (as this is currently a private method, we may need to use Reflection or the [PrivateObject](https://docs.microsoft.com/en-us/dotnet/api/microsoft.visualstudio.testtools.unittesting.privateobject.-ctor?view=mstest-net-1.3.2) functionality for testing)
- [x] Unit tests for the entire SignCheckTask class