NUnit tests are using deprecated asserts. Migrate to new NUnit patterns.

[AtwoodTM]

See:

https://github.com/nunit/nunit.analyzers/tree/master/documentation/NUnit2005.md https://github.com/nunit/nunit.analyzers/blob/master/documentation/NUnit2006.md

These migrations need to be made to remove warnings in .NET 6 and to allow upgrading to .NET 8 (where they are errors).