[SPARK-6005][TESTS] Fix flaky test: o.a.s.streaming.kafka.DirectKafkaStreamSuite.offset recovery

## What changes were proposed in this pull request?

Because this test extracts data from `DStream.generatedRDDs` before stopping, it may get data before checkpointing. Then after recovering from the checkpoint, `recoveredOffsetRanges` may contain something not in `offsetRangesBeforeStop`, which will fail the test. Adding `Thread.sleep(1000)` before `ssc.stop()` will reproduce this failure.

This PR just moves the logic of `offsetRangesBeforeStop` (also renamed to `offsetRangesAfterStop`) after `ssc.stop()` to fix the flaky test.

## How was this patch tested?

Jenkins unit tests.

Author: Shixiong Zhu <shixiong@databricks.com>

Closes #12903 from zsxwing/SPARK-6005.