[SPARK-18617][SPARK-18560][TESTS] Fix flaky test: StreamingContextSuite....

[SPARK-18617][SPARK-18560][TESTS] Fix flaky test: StreamingContextSuite. Receiver data should be deserialized properly

## What changes were proposed in this pull request?

Avoid to create multiple threads to stop StreamingContext. Otherwise, the latch added in #16091 can be passed too early.

## How was this patch tested?

Jenkins

Author: Shixiong Zhu <shixiong@databricks.com>

Closes #16105 from zsxwing/SPARK-18617-2.