[SPARK-20751][SQL][FOLLOWUP] Add cot test in MathExpressionsSuite

## What changes were proposed in this pull request?

Add cot test in MathExpressionsSuite as https://github.com/apache/spark/pull/17999#issuecomment-302832794.

## How was this patch tested?

unit tests

Author: Yuming Wang <wgyumg@gmail.com>

Closes #18039 from wangyum/SPARK-20751-test.