Update to support NUnit 4.x

[david-obee]

NUnit 4.0.0 moves out support for the classic assertion style (over constraint assertions) to another namespace: https://docs.nunit.org/articles/nunit/release-notes/framework.html#the-following-issues-are-marked-as-breaking-changes

This is incompatible for this library, e.g.:

System.TypeLoadException : Could not load type 'NUnit.Framework.CollectionAssert' from assembly 'nunit.framework, Version=4.0.1.0, Culture=neutral, PublicKeyToken=2638cd05610744eb'.

[david-obee]

I'll raise a PR for this