MSTest (the default testing framework) requires you to have your test classes decorated by a [TestClass] attribute, and the test methods with a [TestMethod] attribute, and to be public.

[TestClass]
public class FizzBuzzFixture
{
    [TestMethod]
    public void Test1()
    {
        //arrange
        var solver = new FizzBuzzSolver();
        //act
        var result = solver.FizzBuzz(1);
        //assert
        Assert.AreEqual("1",result);
    }
}