[SPARK-13419] [SQL] Update SubquerySuite to use checkAnswer for validation

## What changes were proposed in this pull request?

Change SubquerySuite to validate test results utilizing checkAnswer helper method

## How was this patch tested?

Existing tests

Author: Luciano Resende <lresende@apache.org>

Closes #12269 from lresende/SPARK-13419.

sql/core/src/test/scala/org/apache/spark/sql/SubquerySuite.scala

@@ -55,32 +55,37 @@ class SubquerySuite extends QueryTest with SharedSQLContext {
   }
 
   test("simple uncorrelated scalar subquery") {
-    assertResult(Array(Row(1))) {
-      sql("select (select 1 as b) as b").collect()
-    }
+    checkAnswer(
+      sql("select (select 1 as b) as b"),
+      Array(Row(1))
+    )
 
-    assertResult(Array(Row(3))) {
-      sql("select (select (select 1) + 1) + 1").collect()
-    }
+    checkAnswer(
+      sql("select (select (select 1) + 1) + 1"),
+      Array(Row(3))
+    )
 
     // string type
-    assertResult(Array(Row("s"))) {
-      sql("select (select 's' as s) as b").collect()
-    }
+    checkAnswer(
+      sql("select (select 's' as s) as b"),
+      Array(Row("s"))
+    )
   }
 
   test("uncorrelated scalar subquery in CTE") {
-    assertResult(Array(Row(1))) {
+    checkAnswer(
       sql("with t2 as (select 1 as b, 2 as c) " +
         "select a from (select 1 as a union all select 2 as a) t " +
-        "where a = (select max(b) from t2) ").collect()
-    }
+        "where a = (select max(b) from t2) "),
+      Array(Row(1))
+    )
   }
 
   test("uncorrelated scalar subquery should return null if there is 0 rows") {
-    assertResult(Array(Row(null))) {
-      sql("select (select 's' as s limit 0) as b").collect()
-    }
+    checkAnswer(
+      sql("select (select 's' as s limit 0) as b"),
+      Array(Row(null))
+    )
   }
 
   test("runtime error when the number of rows is greater than 1") {
@@ -88,29 +93,34 @@ class SubquerySuite extends QueryTest with SharedSQLContext {
       sql("select (select a from (select 1 as a union all select 2 as a) t) as b").collect()
     }
     assert(error2.getMessage.contains(
-      "more than one row returned by a subquery used as an expression"))
+      "more than one row returned by a subquery used as an expression")
+    )
   }
 
   test("uncorrelated scalar subquery on a DataFrame generated query") {
     val df = Seq((1, "one"), (2, "two"), (3, "three")).toDF("key", "value")
     df.registerTempTable("subqueryData")
 
-    assertResult(Array(Row(4))) {
-      sql("select (select key from subqueryData where key > 2 order by key limit 1) + 1").collect()
-    }
+    checkAnswer(
+      sql("select (select key from subqueryData where key > 2 order by key limit 1) + 1"),
+      Array(Row(4))
+    )
 
-    assertResult(Array(Row(-3))) {
-      sql("select -(select max(key) from subqueryData)").collect()
-    }
+    checkAnswer(
+      sql("select -(select max(key) from subqueryData)"),
+      Array(Row(-3))
+    )
 
-    assertResult(Array(Row(null))) {
-      sql("select (select value from subqueryData limit 0)").collect()
-    }
+    checkAnswer(
+      sql("select (select value from subqueryData limit 0)"),
+      Array(Row(null))
+    )
 
-    assertResult(Array(Row("two"))) {
+    checkAnswer(
       sql("select (select min(value) from subqueryData" +
-        " where key = (select max(key) from subqueryData) - 1)").collect()
-    }
+        " where key = (select max(key) from subqueryData) - 1)"),
+      Array(Row("two"))
+    )
   }
 
   test("EXISTS predicate subquery") {