Upgrade NUnit to 4.x

[m-redding]

Migrate the entire repository following the migration guide found at https://docs.nunit.org/articles/nunit/release-notes/Nunit4.0-MigrationGuide.html

The repository is currently built on NUnit 3. There may be additional dependency bumps that need to be made as part of this work. These dependency changes should all be done in Packages.Data.Props.

All test projects across the repository will likely need to be updated.