test/class_info: fix failing test in Lua 5.2

Apparently, with Lua 5.2 the order of the attributes is not preserved
and may change at random, causing failing tests in test_class_info, see
for example:

http://my.cdash.org/testDetails.php?test=12869358&build=691599

    halmd/master/src/libs/luaponte/test/test_class_info.cpp:57"ERROR:
    "[string "x = X()..."]:6: assertion failed!""