Exercise: The quote service

Now it is your turn to use what you've learned and write tests to cover the remaining use cases.

The tests needed are:

• GetQuoteAsync_ShouldThrowNegativeAmountException_WhenAmountIsZeroOrNegative
• GetQuoteAsync_ShouldReturnNull_WhenNoRateExists
• GetQuoteAsync_ShouldReturnQuote_WhenCurrenciesAreValid - Rewrite this test to use [Theory] and multiple parameters