[SPARK-12026][MLLIB] ChiSqTest gets slower and slower over time when number of features is large

jira: https://issues.apache.org/jira/browse/SPARK-12026

The issue is valid as features.toArray.view.zipWithIndex.slice(startCol, endCol) becomes slower as startCol gets larger.

I tested on local and the change can improve the performance and the running time was stable.

Author: Yuhao Yang <hhbyyh@gmail.com>

Closes #10146 from hhbyyh/chiSq.