Add Audiobooks tests

[FC5570]

Currently, audibooks are only available in the US, so I am not able to access any to get the ID and test. If anyone is willing to write tests for the Audiobooks, please comment here and I will assign you.

Take a look at tests to familiarize yourself with the code. Then create a similar test for both the manager's functions (fetch and fetchSeveral)

[prashanthr]

Hi @FC5570, I can help out with this issue.

Some background: I've previously used and spent enough time using the unofficial NodeJS based spotify wrapper - https://github.com/thelinmichael/spotify-web-api-node.

You can assign this to me and I'll open a PR to add these tests. Thanks.