Add Stopwatch to ExecuteCases.cs

public class ExecuteCases : InstanceBehavior
{
    public void Execute(Fixture fixture)
    {
        foreach (var @case in fixture.Cases)
        {
            var stopwatch = new Stopwatch();
            stopwatch.Start();

            try
            {
                fixture.CaseExecutionBehavior.Execute(@case, fixture.Instance);
            }
            catch (Exception exception)
            {
                @case.Fail(exception);
            }

            stopwatch.Stop();

            @case.Duration = stopwatch.Elapsed;
        }
    }
}