[SPARK-15740][MLLIB] Word2VecSuite "big model load / save" caused OOM in maven jenkins builds

## What changes were proposed in this pull request?
"test big model load / save" in Word2VecSuite, lately resulted into OOM.
Therefore we decided to make the partitioning adaptive (not based on spark default "spark.kryoserializer.buffer.max" conf) and then testing it using a small buffer size in order to trigger partitioning without allocating too much memory for the test.

## How was this patch tested?
It was tested running the following unit test:
org.apache.spark.mllib.feature.Word2VecSuite

Author: tmnd1991 <antonio.murgia2@studio.unibo.it>

Closes #13509 from tmnd1991/SPARK-15740.