[SPARK-18296][SQL] Use consistent naming for expression test suites

## What changes were proposed in this pull request?
We have an undocumented naming convention to call expression unit tests ExpressionsSuite, and the end-to-end tests FunctionsSuite. It'd be great to make all test suites consistent with this naming convention.

## How was this patch tested?
This is a test-only naming change.

Author: Reynold Xin <rxin@databricks.com>

Closes #15793 from rxin/SPARK-18296.

sql/catalyst/src/test/scala/org/apache/spark/sql/catalyst/expressions/BitwiseFunctionsSuite.scala → sql/catalyst/src/test/scala/org/apache/spark/sql/catalyst/expressions/BitwiseExpressionsSuite.scala

@@ -21,7 +21,7 @@ import org.apache.spark.SparkFunSuite
 import org.apache.spark.sql.types._
 
 
-class BitwiseFunctionsSuite extends SparkFunSuite with ExpressionEvalHelper {
+class BitwiseExpressionsSuite extends SparkFunSuite with ExpressionEvalHelper {
 
   import IntegralLiteralTestUtils._
 

sql/catalyst/src/test/scala/org/apache/spark/sql/catalyst/expressions/CollectionFunctionsSuite.scala → sql/catalyst/src/test/scala/org/apache/spark/sql/catalyst/expressions/CollectionExpressionsSuite.scala

@@ -20,8 +20,7 @@ package org.apache.spark.sql.catalyst.expressions
 import org.apache.spark.SparkFunSuite
 import org.apache.spark.sql.types._
 
-
-class CollectionFunctionsSuite extends SparkFunSuite with ExpressionEvalHelper {
+class CollectionExpressionsSuite extends SparkFunSuite with ExpressionEvalHelper {
 
   test("Array and Map Size") {
     val a0 = Literal.create(Seq(1, 2, 3), ArrayType(IntegerType))

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

@@ -29,7 +29,7 @@ import org.apache.spark.sql.catalyst.optimizer.SimpleTestOptimizer
 import org.apache.spark.sql.catalyst.plans.logical.{OneRowRelation, Project}
 import org.apache.spark.sql.types._
 
-class MathFunctionsSuite extends SparkFunSuite with ExpressionEvalHelper {
+class MathExpressionsSuite extends SparkFunSuite with ExpressionEvalHelper {
 
   import IntegralLiteralTestUtils._
 

sql/catalyst/src/test/scala/org/apache/spark/sql/catalyst/expressions/MiscFunctionsSuite.scala → sql/catalyst/src/test/scala/org/apache/spark/sql/catalyst/expressions/MiscExpressionsSuite.scala

@@ -20,7 +20,7 @@ package org.apache.spark.sql.catalyst.expressions
 import org.apache.spark.SparkFunSuite
 import org.apache.spark.sql.types._
 
-class MiscFunctionsSuite extends SparkFunSuite with ExpressionEvalHelper {
+class MiscExpressionsSuite extends SparkFunSuite with ExpressionEvalHelper {
 
   test("assert_true") {
     intercept[RuntimeException] {

sql/catalyst/src/test/scala/org/apache/spark/sql/catalyst/expressions/NullFunctionsSuite.scala → sql/catalyst/src/test/scala/org/apache/spark/sql/catalyst/expressions/NullExpressionsSuite.scala

@@ -23,7 +23,7 @@ import org.apache.spark.sql.catalyst.expressions.objects.AssertNotNull
 import org.apache.spark.sql.catalyst.plans.logical.{LocalRelation, Project}
 import org.apache.spark.sql.types._
 
-class NullFunctionsSuite extends SparkFunSuite with ExpressionEvalHelper {
+class NullExpressionsSuite extends SparkFunSuite with ExpressionEvalHelper {
 
   def testAllTypes(testFunc: (Any, DataType) => Unit): Unit = {
     testFunc(false, BooleanType)

sql/core/src/test/scala/org/apache/spark/sql/MathExpressionsSuite.scala → sql/core/src/test/scala/org/apache/spark/sql/MathFunctionsSuite.scala

@@ -23,13 +23,13 @@ import org.apache.spark.sql.functions._
 import org.apache.spark.sql.functions.{log => logarithm}
 import org.apache.spark.sql.test.SharedSQLContext
 
-private object MathExpressionsTestData {
+private object MathFunctionsTestData {
   case class DoubleData(a: java.lang.Double, b: java.lang.Double)
   case class NullDoubles(a: java.lang.Double)
 }
 
-class MathExpressionsSuite extends QueryTest with SharedSQLContext {
-  import MathExpressionsTestData._
+class MathFunctionsSuite extends QueryTest with SharedSQLContext {
+  import MathFunctionsTestData._
   import testImplicits._
 
   private lazy val doubleData = (1 to 10).map(i => DoubleData(i * 0.2 - 1, i * -0.2 + 1)).toDF()