filechooserbutton: Add a description string to assertions about emitted signals
filechooserbutton: Add a description string to assertions about emitted signals

On failed tests, this lets us more easily see see which assertion failed.

Signed-off-by: Federico Mena Quintero <4999915a961edfd7686112c2935288e1266eae14@gnome.org>

Distrotech/gtk,johne53/MB3Gtk-2,johne53/MB3Gtk-2,johne53/MB3Gtk-2,johne53/MB3Gtk-2,Distrotech/gtk,Distrotech/gtk,Distrotech/gtk,johne53/MB3Gtk-2,Distrotech/gtk