Add the annotation @GenerateMocks([http.Client]) to the main
function to generate a MockClient class with mockito.

The generated MockClient class implements the http.Client class.
This allows you to pass the MockClient to the fetchAlbum function,
and return different http responses in each test.

The generated mocks will be located in fetch_album_test.mocks.dart.
Import this file to use them.

Next, generate the mocks running the following command:

flutter pub run build_runner build

4. Write a test for each condition

The fetchAlbum() function does one of two things:

Returns an Album if the http call succeeds

Throws an Exception if the http call fails