test-completion-model: test set_visible_providers()
test-completion-model: test set_visible_providers()

It shows other bugs in CompletionModel (the commented tests). It's
difficult to understand how the UI is not broken.

GNOME/gtksourceview,cburschka/gtksourceview,cburschka/gtksourceview,GNOME/gtksourceview,GNOME/gtksourceview,cburschka/gtksourceview,GNOME/gtksourceview,cburschka/gtksourceview,GNOME/gtksourceview,GNOME/gtksourceview,GNOME/gtksourceview,uajain/gtksourceview,GNOME/gtksourceview,GNOME/gtksourceview,uajain/gtksourceview,cburschka/gtksourceview,uajain/gtksourceview,GNOME/gtksourceview,uajain/gtksourceview,GNOME/gtksourceview,uajain/gtksourceview,GNOME/gtksourceview,uajain/gtksourceview,cburschka/gtksourceview,uajain/gtksourceview,uajain/gtksourceview,uajain/gtksourceview,cburschka/gtksourceview,cburschka/gtksourceview,GNOME/gtksourceview,cburschka/gtksourceview,uajain/gtksourceview,GNOME/gtksourceview,cburschka/gtksourceview,GNOME/gtksourceview