NUnit: migrate to constraints model

[metoule]

Starting with NUnit 4, the "classic" model of assertions is no longer available. This PR updates the test dependencies to the latest versions, including NUnit, and use the new constraint model.

This means that instead of writing:

Assert.AreEqual(23, target.Eval("myVar"));

we must write

Assert.That(target.Eval("myVar"), Is.EqualTo(23));

Note that the goal is to have up-to-date dependencies, but we can stay on the 3.x version if you don't like the constraint model :)

[davideicardi]

Thank you!

I admin that the new syntax looks a little bit "unusual" for me, but using legacy code/syntax is never a good idea. LGTM!