tests: return 0; from main()

So that gcc stops complaining.

gtk/tests/accessible.c

@@ -33,4 +33,6 @@ main (int argc, char *argv[])
 
   for (i = 0; tp[i]; i++)
     test_type (tp[i]);
+
+  return 0;
 }

tests/testgeometry.c

@@ -219,4 +219,6 @@ main(int argc, char **argv)
   create_window (GDK_HINT_RESIZE_INC | GDK_HINT_BASE_SIZE | GDK_HINT_MIN_SIZE);
 
   gtk_main ();
+
+  return 0;
 }

tests/testtoplevelembed.c

@@ -100,4 +100,6 @@ main (gint argc, gchar **argv)
 
   gtk_widget_show_all (window);
   gtk_main ();
+
+  return 0;
 }