[test] Fix compiler warning on older gcc versions

Reported-by: Alex Davies <alex@davz.net>
Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/tests/list_test.c

 	pos = &list_tests[7];
 	list_iterate_entry_ok ( list_for_each_entry_continue, "293",
 				pos, list, list );
-	pos = list_entry ( list, struct list_test, list );
+	ok ( pos == list_entry ( list, struct list_test, list ) );
 	list_iterate_entry_ok ( list_for_each_entry_continue, "47293",
 				pos, list, list );
 	pos = &list_tests[3];
 	pos = &list_tests[2];
 	list_iterate_entry_ok ( list_for_each_entry_continue_reverse, "74",
 				pos, list, list );
-	pos = list_entry ( list, struct list_test, list );
+	ok ( pos == list_entry ( list, struct list_test, list ) );
 	list_iterate_entry_ok ( list_for_each_entry_continue_reverse, "39274",
 				pos, list, list );
 	pos = &list_tests[4];