test: fix flaky redis unit tests

[Uzlopak]

We were running the unit tests in a very flaky way. Instead of actually waiting the redis flushall command we were having a noop callback. So it could happen, that flushall was not done and we were already running the next test.

This PR fixes that by implementing a teardown for those redis tests and fixing the corresponding assertions.

Checklist

• run npm run test and npm run benchmark
• tests and/or benchmarks are included
• documentation is changed or added
• commit message and code follows the Developer's Certification of Origin
  and the Code of conduct

[climba03003]

LGTM. Let wait one more people before merging.