[SPARK-18954][TESTS] Fix flaky test: o.a.s.streaming.BasicOperationsSuite rdd...

[SPARK-18954][TESTS] Fix flaky test: o.a.s.streaming.BasicOperationsSuite rdd cleanup - map and window

## What changes were proposed in this pull request?

The issue in this test is the cleanup of RDDs may not be able to finish before stopping StreamingContext. This PR basically just puts the assertions into `eventually` and runs it before stopping StreamingContext.

## How was this patch tested?

Jenkins

Author: Shixiong Zhu <shixiong@databricks.com>

Closes #16362 from zsxwing/SPARK-18954.

(cherry picked from commit 078c71c2)
Signed-off-by: Shixiong Zhu <shixiong@databricks.com>