Migrate to Nunit4

[jnyrup]

The first commit migrates from from NUnit3 to NUnit4.

• Towards NUnit Version 4 explains what the new syntax offers.
• Migration Guidance mentions breaking changes from NUnit3.

All the changes to the new Assert.That syntax (Called the constraint model) where done automatically by the added NUnit.Analyzers.
Their list of analyzers.

In the second commit I went through the warnings from NUnit1028 which warns about public methods in test classes that are not marked with [Test].
I've blindly added [Test] to all those that had asserts in them.

[kblok]

All the changes to the new Assert.That syntax (Called the constraint model) where done automatically by the added NUnit.Analyzers.

I'm glad that you didn't have to do that manually!

[kblok]

I would try to land this soon. And we could work in some improvements later.
I don't want to conflict this PR on every change.

[jnyrup]

@kblok the new NUnit analyzers pointed out methods that contained assertions, but weren't annotated with [Test].
I've added those annotations in 9e18877 but I could use help figuring out how they should be annotated.

We could cherry-pick that commit to another PR with other changes necessary to have them pass on master as well.

[kblok]

@jnyrup do you think we can land this?

[jnyrup]

I don't have any pending changes.

There's a failing tests with an NRE, I haven't looked into the cause of that, but I guess it's unrelated to this PR.