[SPARK-16251][SPARK-20200][CORE][TEST] Flaky test: org.apache.spark.rdd.LocalCheckpointSuite.missing checkpoint block fails with informative message

## What changes were proposed in this pull request?

Currently we don't wait to confirm the removal of the block from the slave's BlockManager, if the removal takes too much time, we will fail the assertion in this test case.
The failure can be easily reproduced if we sleep for a while before we remove the block in BlockManagerSlaveEndpoint.receiveAndReply().

## How was this patch tested?
N/A

Author: Xingbo Jiang <[email protected]>

Closes apache#18314 from jiangxb1987/LocalCheckpointSuite.

(cherry picked from commit 7dc3e69)
Signed-off-by: Wenchen Fan <[email protected]>

Author: jiangxb1987

core/src/test/scala/org/apache/spark/rdd/LocalCheckpointSuite.scala

@@ -17,6 +17,10 @@
 
 package org.apache.spark.rdd
 
+import scala.concurrent.duration._
+
+import org.scalatest.concurrent.Eventually.{eventually, interval, timeout}
+
 import org.apache.spark.{LocalSparkContext, SparkContext, SparkException, SparkFunSuite}
 import org.apache.spark.storage.{RDDBlockId, StorageLevel}
 
@@ -168,6 +172,10 @@ class LocalCheckpointSuite extends SparkFunSuite with LocalSparkContext {
     // Collecting the RDD should now fail with an informative exception
     val blockId = RDDBlockId(rdd.id, numPartitions - 1)
     bmm.removeBlock(blockId)
+    // Wait until the block has been removed successfully.
+    eventually(timeout(1 seconds), interval(100 milliseconds)) {
+      assert(bmm.getBlockStatus(blockId).isEmpty)
+    }
     try {
       rdd.collect()
       fail("Collect should have failed if local checkpoint block is removed...")