Fix make warnings since gtk2 drop
- Fix duplicate entries in tests/Makefile.am
- We need to include glib/gi18n-lib.h when GETTEXT_PACKAGE is defined, not glib/gi18n.h
- "state' variable is a GtkStateFlags, GtkStateType is deprecated since gtk 3.14, and we are using GTK_STATE_FLAG_* anyway
Edited by Romain Bouvier