Dangling CountDownLatch.await() in tests

There are several places in tests where the boolean result of `CountDownLatch.await(timeout)` is not tested. It should properly be wrapped into `assertTrue()` in (almost) all cases. Currently, these tests don't really verify the correctness of the production code.

See https://github.com/code-review-checklists/java-concurrency#check-await

To systematically eradicate these bugs, either the configuration for "Result of method call is ignored" inspection should be edited and the inspection is enabled on error level (which I think is preferable because we want to strictly check this inspection anyway), or a separate [Structural Search pattern](https://github.com/apache/druid/blob/master/dev/teamcity.md#creating-a-custom-inspection-from-a-structural-search-pattern) could be easily created.

Search the tests for CountDownLatch.await(timeout) calls whose boolean result is ignored, then read the IntelliJ "Result of method call is ignored" inspection configuration and the structural-search guidance in dev/teamcity.md. Done means the affected test calls assert their result and a chosen inspection or search rule catches remaining cases.

Written by the indexing model from the issue text.