Improve multiline labeling for tooltips in doughnut charts

[etimberg]

Change the default doughnut chart tooltip callback to better handle multiline labels. I put the value on the first line. I think this is fine since it matches how the color box is drawn in the label.

Fixes #3269

After Changes

[simonbrunel]

Looks good. I think we could even get rid of defaultConfig.tests.js and move these tests next to the associated classes (e.g. bubble chart default config in controller.bubble.tests.js, etc.). If it make sense, we can do that later of course.

[etimberg]

yeah, these are becoming much more chart specific so we can move them into the controller tests at some point