[Test] // everything works
public void TestGetPriceNormalValues()
{
//arrange
string symbol = "aapl";
_stockApi.GetPrice(symbol).Returns(23.21);
//act
double price = _stockApi.GetPrice(symbol);
_trader.Buy(symbol, price);
//assert
_trader.Received().Buy(Arg.Any <string>(), Arg.Any <double>());
}