How do I put new List<int> {1} in an NUNIT TestCase?

There is one option to use TestCaseSource attribute. Here I provide a non-assert test with two cases just to see how it works:

[TestFixture]
public class TestClass
{
    private static readonly object[] _sourceLists = 
    {
        new object[] {new List<int> {1}},   //case 1
        new object[] {new List<int> {1, 2}} //case 2
    };

    [TestCaseSource("_sourceLists")]
    public void Test(List<int> list)
    {
        foreach (var item in list)
            Console.WriteLine(item);
    }
}

Anyhow I have to mention it is not the most evident solution and I would prefer neatly organized fixtures ignoring the fact they are more verbose

More information: https://github.com/nunit/docs/wiki/TestCaseSource-Attribute

My solution is simpler, I just use params. I hope this works for you!

[TestCase(1, 1)]
[TestCase(10, 5, 1, 4)]
[TestCase(25, 3, 5, 5, 12)]
public void Linq_Add_ShouldSumAllTheNumbers(int expected, params int[] numbers)
{
    var result = CalculatorLibrary.CalculatorFunctions.Add(numbers);
    Assert.AreEqual(expected, result);
}

I often use strings and parsing as it renders nicely in the testrunner. Sample:

[TestCase("1, 2")]
[TestCase("1, 2, 3")]
public void WithStrings(string listString)
{
    var list = listString.Split(',')
                         .Select(int.Parse)
                         .ToList();
    ...
}

Looks like this in Resharper's runner: