utils: Use ISO weekday numbering for 'get_first_weekday' function

Change the 'get_first_weekday' function to number the weekdays
Monday(1)-Sunday(7) like ISO does. Change the function name to reflect
that. Add tests for different locales, although the tests depend on
system-installed locales.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-calendar/-/merge_requests/822>

src/gui/common/gcal-date-chooser.c

@@ -1083,7 +1083,7 @@ gcal_date_chooser_init (GcalDateChooser *self)
   self->date = g_date_time_new_now_local ();
   g_date_time_get_ymd (self->date, &self->this_year, NULL, NULL);
 
-  self->week_start = gcal_util_get_first_weekday ();
+  self->week_start = gcal_util_get_first_weekday_iso ();
 
   gtk_widget_init_template (GTK_WIDGET (self));