[SPARK-15033][SQL] fix a flaky test in CachedTableSuite

## What changes were proposed in this pull request?

This is caused by https://github.com/apache/spark/pull/12776, which removes the `synchronized` from all methods in `AccumulatorContext`.

However, a test in `CachedTableSuite` synchronize on `AccumulatorContext` and expecting no one else can change it, which is not true anymore.

This PR update that test to not require to lock on `AccumulatorContext`.

## How was this patch tested?

N/A

Author: Wenchen Fan <wenchen@databricks.com>

Closes #12811 from cloud-fan/flaky.

sql/core/src/main/scala/org/apache/spark/sql/execution/columnar/InMemoryTableScanExec.scala

@@ -68,7 +68,7 @@ private[sql] case class InMemoryRelation(
 
   override def producedAttributes: AttributeSet = outputSet
 
-  private val batchStats: Accumulable[ArrayBuffer[InternalRow], InternalRow] =
+  private[sql] val batchStats: Accumulable[ArrayBuffer[InternalRow], InternalRow] =
     if (_batchStats == null) {
       child.sqlContext.sparkContext.accumulableCollection(ArrayBuffer.empty[InternalRow])
     } else {

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

@@ -333,12 +333,19 @@ class CachedTableSuite extends QueryTest with SQLTestUtils with SharedSQLContext
     sql("SELECT * FROM t1").count()
     sql("SELECT * FROM t2").count()
 
-    AccumulatorContext.synchronized {
-      val accsSize = AccumulatorContext.numAccums
-      sqlContext.uncacheTable("t1")
-      sqlContext.uncacheTable("t2")
-      assert((accsSize - 2) == AccumulatorContext.numAccums)
-    }
+    val accId1 = sqlContext.table("t1").queryExecution.withCachedData.collect {
+      case i: InMemoryRelation => i.batchStats.id
+    }.head
+
+    val accId2 = sqlContext.table("t1").queryExecution.withCachedData.collect {
+      case i: InMemoryRelation => i.batchStats.id
+    }.head
+
+    sqlContext.uncacheTable("t1")
+    sqlContext.uncacheTable("t2")
+
+    assert(AccumulatorContext.get(accId1).isEmpty)
+    assert(AccumulatorContext.get(accId2).isEmpty)
   }
 
   test("SPARK-10327 Cache Table is not working while subquery has alias in its project list") {