This test has a word that wouldn't fit in the room we have with
a window width of 100, and then we get clipped in the one case
but not the other. Make the window a little wider, so it fits.
I found that setting margins to zero makes the textview clip
away some overshooting pixels. So, instead of a 0/10 split,
do this test with a 2/8 split of margins, to avoid the clipping
issue.
add a reftest that checks GtkTextView text margin property comparing
with the normal margin added to the widget (they are the same when there
is nothing drawn in the gutter like line numbers etc).
