Why should you avoid conditional logic in unit tests and how?

Imagine having the following classes:

public class Product {
   private String name;
   private double price;

   // Constructors, getters and setters
}

public class Products {
   private List<Product> products;

   // CRUD methods

   public double getTotalPrice() {
      // calculates the price of all products
   }
}

I have read that conditional logic(if's and loops) should be avoided in unit tests, but didn't understand exactly why and more importantly how. How can I efficiently test the scenario where I add some products in Products with different prices and then verify the result of getTotalPrice() without using loops?