KAFKA-8098: Fix Flaky Test testConsumerGroups

- The flaky failure is caused by the fact that the main thread sometimes issues DescribeConsumerGroup request before the consumer assignment takes effect. Added a latch to make sure such situation is not going to happen.

Author: huxihx <huxi_2b@hotmail.com>
Author: huxi <huxi_2b@hotmail.com>
Author: Manikumar Reddy <manikumar.reddy@gmail.com>

Reviewers: Manikumar Reddy <manikumar.reddy@gmail.com>

Closes #6441 from huxihx/KAFKA-8098