gtk3-gi: add Gtk::GestureSingle tests

Since GTK+3 3.14 GtkGestureSingle is supported.
ref: https://developer.gnome.org/gtk3/3.14/GtkGestureSingle.html
gtk3-gi: add Gtk::GestureMultiPress tests

TODO:
Gtk::GestureMultiPress#area? returns slightly different pointer:

<[true, #<Gdk::Rectangle:0x7fc9260c3ba8 ptr=0x7fc925d67c00 own=true>]> expected but was
<[true, #<Gdk::Rectangle:0x7fc924ab1360 ptr=0x7fc925d6d1e0 own=true>]>
gtk3-gi: add Gtk::GesturePan tests
gtk3-gi: move Gtk::Gesture#window test into TestGtkGesture