[Infrastructure] - Create central long-text test content and layout

[matthew-carroll]

In #1489 I used a specific layout of a long paragraph of text to test long-press dragging behavior, which selects by word and by character.

Consider establishing this layout as a central, re-usable layout for any such tests. If we do that, keep a reference golden test along with the document content, the physical test size, and the pixel ratio. Also, document the text positions for every start and end of each line. Add a non-golden test that expects every text position at the beginning and end of each line to make sure it doesn't shift over time.