gtk3 test: fix meaningless assertion

`@stack.signal_emit("child-notify", nil)` should not be called by
itself. If these tests call it by itself,
`@stack.signal_connect("child-notify")` is always true. So these tests
don't check anything.
gtk3 test: suppress a warning

    Gtk-WARNING **:Given child of type 'GtkEventBox' not found in GtkStack
gtk3 test: use more concrete value for the expected value

By the way, the test should check whether the widget is added or not
rather than return value. Adding is the important feature of the
`#add` method.
gtk3 test: group tests for #add
gtk3 test: indent
gtk3 test: share common code
gtk3 test: use more meaningful test names
gtk3 test: add a test for adding
gtk3: fix style

"*" should be placed at variable side.
gtk3: add a missing space before "("
gtk3: fix indent level
gtk3: use super's return value
gtk3 test: group set_visible_child test
gtk3 test: indent
gtk3 test: share common code
gtk3: implement Gtk::Revealer

GtkRevealer is provided in Gtk+3 3.10 or later.
gtk3 test: add Gtk::Revealer tests