Testing of interactive plotting features

PR #700 introduced the option hover_label, which shows the label of a datapoint when the user hovers over it with the mouse. For now, this feature is not tested in CI, because it requires an interactive backend.

Thoughts on implementation

Add one extra job skipping all tests except the interactive ones? It would require to somehow fake the hover action too, as part of the code is only executed when the hover action is triggered.