Add `DateTime`/`TimeSpan` support for inequality tolerance

[RenderMichael]

It would be nice if the following code worked:

public static DateTime LastDateAccessed { get; set; } // simulate a remote value

[Test]
public void TestMe()
{
    DateTime beforeUpdate = DateTime.UtcNow;

    // Do operation
    LastDateAccessed = DateTime.UtcNow.AddMilliseconds(-50); // some kind of timing imperfection on the server

    // This fails because of the timing issue
    Assert.That(LastDateAccessed, Is.GreaterThan(beforeUpdate));

    // It would be nice to do this
    Assert.That(LastDateAccessed, Is.GreaterThan(beforeUpdate).Within(TimeSpan.FromSeconds(1)));
}

For consistency, this should probably be done on all constraints that extend ComparisonConstraint.

I am willing to take up the task. I just want to make sure this is kind of change would be accepted.

[RenderMichael]

My workaround, using the above example:

Assert.That(LastDateAccessed, Is.InRange(beforeUpdate - TimeSpan.FromSeconds(1), beforeUpdate + TimeSpan.FromSeconds(1)));

But it's less elegant or intuitive.

[manfred-brands]

@RenderMichael The ComparisonConstraint does seem to have support for Within, it might just be that it doesn't work on DateTime. The special time suffixes are only on the EqualsConstraint. Maybe those need to be move up to its base class. It also looks like the Tolerance.LinearRange doesn't handle this.

Feel free to work on this to make it more consistent with IsEqualTo.

[RenderMichael]

Opened PR #4618

[OsirisTerje]

@RenderMichael Can you please check this package https://www.myget.org/feed/nunit/package/nuget/NUnit/4.1.0-alpha.0.1

[RenderMichael]

Works flawlessly 👍

[OsirisTerje]

@RenderMichael Seems this one needs documentation too, for the constraints that now have got them.

Can you add that?

[RenderMichael]

@OsirisTerje opened nunit/docs#907, let me know if there's something else you had in mind as I've never contributed docs before.