[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.

sql/catalyst/src/test/scala/org/apache/spark/sql/catalyst/expressions/MathExpressionsSuite.scala

@@ -219,6 +219,12 @@ class MathExpressionsSuite extends SparkFunSuite with ExpressionEvalHelper {
     checkConsistencyBetweenInterpretedAndCodegen(Tan, DoubleType)
   }
 
+  test("cot") {
+    def f: (Double) => Double = (x: Double) => 1 / math.tan(x)
+    testUnary(Cot, f)
+    checkConsistencyBetweenInterpretedAndCodegen(Cot, DoubleType)
+  }
+
   test("atan") {
     testUnary(Atan, math.atan)
     checkConsistencyBetweenInterpretedAndCodegen(Atan, DoubleType)