Note that it is not possible for msgpack_unpack_next() and
msgpack_unpacker_next() to return MSGPACK_UNPACK_EXTRA_BYTES, so it
should be fine to abort() on that.
Lua 5.1 doesn't support string hex escapes (\xXX) like VimL does (though
LuaJIT does), so convert them to decimal escapes (\DDD) in tests.