Fix order-dependent test failures in Redis/Sidekiq specs

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.

spec/lib/gitlab/redis/queues_spec.rb

@@ -60,6 +60,10 @@
       described_class.remove_instance_variable(:@instances) if described_class.instance_variable_defined?(:@instances)
     end
 
+    after do
+      described_class.remove_instance_variable(:@instances) if described_class.instance_variable_defined?(:@instances)
+    end
+
     shared_examples 'no extra shards' do
       it 'returns a single map of self' do
         expect(described_class.instances).to eq({ 'main' => described_class })