tests: Return more widgets from create_wibox()

All of these are checked for being GC'able in test-leaks.lua.

Signed-off-by: Uli Schlachter <psychon@znc.in>

tests/_wibox_helper.lua

@@ -28,5 +28,5 @@ return { create_wibox = function()
     --wb.visible = true
     wb:set_widget(layout)
 
-    return wb, textclock
+    return wb, textclock, img, left_layout, right_layout, layout
 end }