[SPARK-7854] [TEST] refine Kryo test suite

this modification is according to JoshRosen 's comments, for details, please refer to [#5934](https://github.com/apache/spark/pull/5934/files#r30949751).

Author: Zhang, Liye <liye.zhang@intel.com>

Closes #6395 from liyezhang556520/kryoTest and squashes the following commits:

da214c8 [Zhang, Liye] refine Kryo test suite accroding to Josh's comments