Hiker (with tests).cs

using NUnit.Framework;

public class Hiker
{
    [TestFixture]
    public class Tests
    {
        [Test]
        public void life_the_universe_and_everything()
        {
            Assert.AreEqual(42, Hiker.Answer());
        }
    }

    public static int Answer()
    {
        return 6 * 9;
    }
}

Hiker.cs

public partial class Hiker
{
    public static int Answer()
    {
        return 6 * 9;
    }
}

Hiker.Tests.cs

using NUnit.Framework;

public partial class Hiker
{
    [TestFixture]
    public class Tests
    {
        [Test]
        public void life_the_universe_and_everything()
        {
            Assert.AreEqual(42, Hiker.Answer());
        }
    }
}