[SPARK-26042][SS][TESTS] Fix a potential hang in KafkaContinuousSourceTopicDeletionSuite

## What changes were proposed in this pull request?

As initializing lazy vals shares the same lock, a thread is trying to initialize `executedPlan` when `isRDD` is running, this thread will hang forever.

This PR just materializes `executedPlan` so that accessing it when `toRdd` is running doesn't need to wait for a lock

## How was this patch tested?

Jenkins

Closes apache#23023 from zsxwing/SPARK-26042.

Authored-by: Shixiong Zhu <[email protected]>
Signed-off-by: Shixiong Zhu <[email protected]>
(cherry picked from commit 4035c98)
Signed-off-by: Shixiong Zhu <[email protected]>

Author: zsxwing

sql/core/src/main/scala/org/apache/spark/sql/execution/streaming/continuous/ContinuousExecution.scala

@@ -259,7 +259,12 @@ class ContinuousExecution(
 
       reportTimeTaken("runContinuous") {
         SQLExecution.withNewExecutionId(
-          sparkSessionForQuery, lastExecution)(lastExecution.toRdd)
+          sparkSessionForQuery, lastExecution) {
+          // Materialize `executedPlan` so that accessing it when `toRdd` is running doesn't need to
+          // wait for a lock
+          lastExecution.executedPlan
+          lastExecution.toRdd
+        }
       }
     } catch {
       case t: Throwable