Add cursor tests for RTL text followed by newline

[valadaptive]

As requested in https://github.com/linebender/parley/issues/298.

It turns out to be a bit worse than I originally thought--when I used ctrl+left arrow in the vello_editor example, it just put me back at the start of the word. But when I call previous_visual_word here, it actually hangs and loops infinitely!

I'm not confident that the assertions for the expected behavior are correct--the assertion helpers seem to be a bit weird. As such, anyone fixing this should not necessarily expect the tests to pass once they've done so.