[SPARK-17102][SQL] bypass UserDefinedGenerator for json format check

## What changes were proposed in this pull request?

We use reflection to convert `TreeNode` to json string, and currently don't support arbitrary object. `UserDefinedGenerator` takes a function object, so we should skip json format test for it, or the tests can be flacky, e.g. `DataFrameSuite.simple explode`, this test always fail with scala 2.10(branch 1.6 builds with scala 2.10 by default), but pass with scala 2.11(master branch builds with scala 2.11 by default).

## How was this patch tested?

N/A

Author: Wenchen Fan <wenchen@databricks.com>

Closes #14679 from cloud-fan/json.

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

@@ -249,9 +249,10 @@ abstract class QueryTest extends PlanTest {
         }
         p
     }.transformAllExpressions {
-      case a: ImperativeAggregate => return
+      case _: ImperativeAggregate => return
       case _: TypedAggregateExpression => return
       case Literal(_, _: ObjectType) => return
+      case _: UserDefinedGenerator => return
     }
 
     // bypass hive tests before we fix all corner cases in hive module.