Make testcellarea handle RTL environment variable for testing

tests/testcellarea.c

@@ -614,6 +614,9 @@ main (int argc, char *argv[])
 {
   gtk_init (NULL, NULL);
 
+  if (g_getenv ("RTL"))
+    gtk_widget_set_default_direction (GTK_TEXT_DIR_RTL);
+
   simple_cell_area ();
   focus_cell_area ();
   background_area ();