[SPARK-20039][ML] rename ChiSquare to ChiSquareTest

## What changes were proposed in this pull request?

I realized that since ChiSquare is in the package stat, it's pretty unclear if it's the hypothesis test, distribution, or what. This PR renames it to ChiSquareTest to clarify this.

## How was this patch tested?

Existing unit tests

Author: Joseph K. Bradley <joseph@databricks.com>

Closes #17368 from jkbradley/SPARK-20039.