Update NUnit to v4

[MrJul]

What does the pull request do?

This PR updates the supported version of NUnit for the headless platform from v3 to v4.
The corresponding unit tests have also been updated.

Notes:

• NUnit 4 is an update mostly compatible with v3, for better and for worse. There still isn't a nice overridable pipeline, but our reflection hacks still work.
• Thread abort timeouts have been removed. They weren't working in modern .NET anyways, and NUnit v4 doesn't support them. This matches our xUnit tests which don't have any timeout.
• An AssertHelper class has been introduced in our unit tests to abstract away from the difference between xUnit and NUnit asserts.

[avaloniaui-bot]

You can test this PR using the following package version. 12.0.999-cibuild0061141-alpha. (feed url: https://nuget-feed-all.avaloniaui.net/v3/index.json) [PRBUILDID]