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

## What changes were proposed in this pull request?

Fixed the potential SparkContext leak in `StreamingContextSuite.SPARK-18560 Receiver data should be deserialized properly` which was added in #16052. I also removed FakeByteArrayReceiver and used TestReceiver directly.

## How was this patch tested?

Jenkins

Author: Shixiong Zhu <shixiong@databricks.com>

Closes #16091 from zsxwing/SPARK-18617-follow-up.