Change the method to take the generic type instance value as shown below
public void AddTestHelper<T>(T item)
{
LinkedList<T> target = new LinkedList<T>();
target.Add(item);
Assert.AreEqual(target.Contains(item), true);
}
so use instance in the code instead
so in your tests u can do
AddTestHelper<int>(12);
AddTestHelper<string>("Test");