public void ChangeIterestTest()
{
Client client = new Client("Jan", "Nowak", "01234567891");
BankAccount bankAccount = new BankAccount("123", new InterestTenPercent(), 1000, client);
bankAccount.AddIterest();
bankAccount.ChangeIterest(new InterestFivePercent());
bankAccount.AddIterest();
Assert.AreEqual(1155, bankAccount.GetSaldo());
}