serversideup
Fix player breaking when `starting_playlist_song is `0`
As mentioned in #407 the player breaks and is unusable, when supplying a starting_playlist_song that is (int) 0.
To alleviate this issue, I adapted the way playlists get initialized and covered my chagnes with tests.
What has changed?
Basically, we now check more closely if the supplied values for starting_playlist and starting_playlist_song are valid. We not check their type and value, but also validate them against the playlist data. So an non existing starting_playlist_song index is as invalid as a NaN for example.
What was adapted to make this change possible?
In order to write tests that could check a larger variety of cases, I needed to adapt the way test data was initalized, so that I could supply my own values. I also adapted the function that resets the configuration after each test in a similar way. But in hind sight that was not necessary.
The newly added test are partially redundant with some existing ones that check the initialization of the playlists. Also I think it might make sense to move all playlist tests from the tests/index.test.js file to the newly added tests/playlist/init.test.js file for an easier to graps structure. But I didn't want to change things around so much without asking. If requested I'll move the playlist tests as mentioned.
More details can be found in the commit messages.
