Add classes to toolbar UI elements
This cleans up the selectors for the elements on the toolbar
so that they can be tested.
Class names in the form of ve-test-* are added to elements
that have words for labels: 'Paragraph', 'Cite', 'Insert'.
There is no other way to look up these elements in the DOM
except using the UI text, but it is different in different languages,
so this is unusable for the language_screenshots job,
which is multilingual by its nature.