The colour of new text in the app was chosen from the page alone, so
on a dark page text drawn in a white frame (a new frame's default) was
white and invisible. readable_text_fills now looks at what the text is
drawn on: the nearest layer from its parent out whose top visible fill
is a mostly opaque solid colour, else the page. The Text tool and
text.create / node.create pass the text's parent.
The two tests that asserted the page-only rule in a frame now expect
black text in a white frame; new cases cover dark, unfilled, nested,
faint and stacked fills, and the Text tool.