tests: wait for draw before fetching tree view style
tests: wait for draw before fetching tree view style

On some slower machines (e.g. an ARM OBS builder), this test is failing
with a race condition where we're trying to fetch the style before it's
applied.

https://bugzilla.gnome.org/show_bug.cgi?id=749593

chergert/gtk,Adamovskiy/gtk,Adamovskiy/gtk,jigpu/gtk,chergert/gtk,grubersjoe/adwaita,grubersjoe/adwaita,jadahl/gtk,Adamovskiy/gtk,Adamovskiy/gtk,Adamovskiy/gtk,jigpu/gtk,davidgumberg/gtk,chergert/gtk,Adamovskiy/gtk,jigpu/gtk,alexlarsson/gtk,jadahl/gtk,jigpu/gtk,davidgumberg/gtk,grubersjoe/adwaita,alexlarsson/gtk,grubersjoe/adwaita,jadahl/gtk,grubersjoe/adwaita,alexlarsson/gtk,davidgumberg/gtk,grubersjoe/adwaita,jadahl/gtk,alexlarsson/gtk,chergert/gtk,alexlarsson/gtk,grubersjoe/adwaita,jigpu/gtk,jigpu/gtk,davidgumberg/gtk,chergert/gtk,davidgumberg/gtk,jadahl/gtk,jadahl/gtk,chergert/gtk,jigpu/gtk,Adamovskiy/gtk,alexlarsson/gtk,chergert/gtk,jadahl/gtk,jadahl/gtk,alexlarsson/gtk,davidgumberg/gtk,Adamovskiy/gtk,chergert/gtk,davidgumberg/gtk,alexlarsson/gtk,jigpu/gtk,grubersjoe/adwaita,davidgumberg/gtk