Upgrade NUnit to 4.x

### Is there an existing issue for this?

- [X] I have searched the existing issues

### Task description

We are on NUnit 3.x, and 4.x is available. To stay current, we should eventually upgrade. However, there is a major blocker at the moment: they have dropped their .NET Standard targets, which means we can't compile TestFramework as-is against NUnit 4.x.

This change also involves some code changes, such as replacing Assert with ClassicAssert (for now; eventually we'll want to get off the "legacy" asserts), but those are pretty easy to do (I did them in just a few minutes locally).

The .NET Standard impediment is the biggest challenge. We would need to decide which targets the TestFramework assembly should support, and if we do this we're limited to those that are compatible with NUnit 4.