Fix order-dependent test failures in Redis/Sidekiq specs

What does this MR do and why?

Previously the second test would fail if you did this:

bundle exec rspec spec/lib/gitlab/redis/queues_spec.rb spec/lib/gitlab/sidekiq_queue_spec.rb

This happens because the @instances class variable is memoized by the first spec with some temporary settings. Clear this out to ensure this doesn't pollute the global state.