Upgrade to NUnit 4 and leverage new features like Assert.ThatAsync and Assert.Multiple

[Bartleby2718]

Nunit 4 was released 4 months ago: https://www.nuget.org/packages/NUnit/4.0.0#versions-body-tab

• Changes: https://docs.nunit.org/articles/nunit/Towards-NUnit4.html
• Migration Guide: https://docs.nunit.org/articles/nunit/release-notes/Nunit4.0-MigrationGuide.html

I think this will improve the developer experience (by showing better error messages) and speed up the test time (by removing sync-over-async).