Update to nunit 4

[jamespicone]

Nunit has changed their assert interface in 4.0; their previous interface is now under some Nunit.Legacy namespace and uses the name ClassicAssert, and they've got a new one where you write Assert.That(thing, Is.EqualTo(blah)) etc.

This patch changes BaseTest to use the ClassicAssert interface and updates the project to depend on nunit4.

[bobbertoriley]

Suggestion: You can reduce the number of changes required to BaseTest by revering Assert -> ClassicAssert and then doing a one-line import. Reduces the diff while maintaining the functionality. It's how I handled the upgrade in my own project.

using Assert = NUnit.Framework.Legacy.ClassicAssert;

If we're going to make changes to the Assert.Foo() lines, it'd probably be better to move them over to NUnit 4 assertions.

[MigrantP]

Is there a pressing need to update to NUnit 4 here?

[jamespicone]

I don't remember the motivation i'm afraid; I had a vague memory that it was an issue with nunit 3's support for more recent .net frameworks but i just checked and it seems fine against 4.8.