[SPARK-21837][SQL][TESTS] UserDefinedTypeSuite Local UDTs not actually testing what it intends

## What changes were proposed in this pull request?

Adjust Local UDTs test to assert about results, and fix index of vector column. See JIRA for details.

## How was this patch tested?

Existing tests.

Author: Sean Owen <sowen@cloudera.com>

Closes #19053 from srowen/SPARK-21837.

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

@@ -203,12 +203,12 @@ class UserDefinedTypeSuite extends QueryTest with SharedSQLContext with ParquetT
 
   // Tests to make sure that all operators correctly convert types on the way out.
   test("Local UDTs") {
-    val df = Seq((1, new UDT.MyDenseVector(Array(0.1, 1.0)))).toDF("int", "vec")
-    df.collect()(0).getAs[UDT.MyDenseVector](1)
-    df.take(1)(0).getAs[UDT.MyDenseVector](1)
-    df.limit(1).groupBy('int).agg(first('vec)).collect()(0).getAs[UDT.MyDenseVector](0)
-    df.orderBy('int).limit(1).groupBy('int).agg(first('vec)).collect()(0)
-      .getAs[UDT.MyDenseVector](0)
+    val vec = new UDT.MyDenseVector(Array(0.1, 1.0))
+    val df = Seq((1, vec)).toDF("int", "vec")
+    assert(vec === df.collect()(0).getAs[UDT.MyDenseVector](1))
+    assert(vec === df.take(1)(0).getAs[UDT.MyDenseVector](1))
+    checkAnswer(df.limit(1).groupBy('int).agg(first('vec)), Row(1, vec))
+    checkAnswer(df.orderBy('int).limit(1).groupBy('int).agg(first('vec)), Row(1, vec))
   }
 
   test("UDTs with JSON") {