Switch to NUnit v4

[rubo]

Changes

Migrated to NUnit v4:

• Replaced Assert.<Method>() assertions with Assert.That() assertions
• Replaced CollectionAssert assertions with Assert assertions
• Replaced TestContext.WriteLine withTestContext.Out.Write
• Replaced obsolete TimeoutAttribute with MaxTimeAttribute

Types of changes

What types of changes does your code introduce?

• Bugfix (a non-breaking change that fixes an issue)
• New feature (a non-breaking change that adds functionality)
• Breaking change (a change that causes existing functionality not to work as expected)
• Optimization
• Refactoring
• Documentation update
• Build-related changes
• Other: Description

Testing

Requires testing

• Yes
• No

If yes, did you write tests?

• Yes
• No

Remarks

With this upgrade, the FluentAssertions package doesn't seem needed.

[LukaszRozmej]

Will CancelAfter fail the test?

[rubo]

Will CancelAfter fail the test?

@LukaszRozmej That's a good question. It won't. Actually, as the Timeout is gone, the replacement is CancelAfter but it works differently. Actually, there are 2 options—MaxTime and CancelAfter. MaxTime fails the test if the test exceeds the time limit. But it lets the test run as long as it takes and then checks the actual duration. CancelAfter is for canceling long-running tasks with a cancellation token, but most of our test APIs don't support that.
With this said, we better use MaxTime instead of CancelAfter.

[LukaszRozmej]

Maybe we should add both?

[rubo]

Maybe we should add both?

In some cases, yes. But our API doesn't support it. Modifying it would bring many cascading changes, which I'd like to avoid in this PR. Another issue is overkill with timeouts. Check this out, for instance. Why do we need a timeout there? There are a bunch of such examples.