Fix order-dependent ActionCable spec failure

If another spec uses ActionCable before the ActionCable initializer
spec runs, then the current PubSub Redis connection will already be
configured with a memoized identifier. The spec attempts to test a
Redis configuration with a `nil` identifier to support GCP
Memorystore, but for that to work we need to reset the Redis
connection and cause the new config to be used.

This commit fixes the order-dependent test by clearing out the
ActionCable server status before and after the test via
`ActionCable.server.restart`.

Closes https://gitlab.com/gitlab-org/gitlab/-/issues/418937

spec/initializers/action_cable_subscription_adapter_identifier_spec.rb

@@ -6,8 +6,13 @@
   describe '#identifier' do
     let!(:original_config) { ::ActionCable::Server::Base.config.cable }
 
+    before do
+      ActionCable.server.restart
+    end
+
     after do
       ::ActionCable::Server::Base.config.cable = original_config
+      ActionCable.server.restart
     end
 
     context 'when id key is nil on cable.yml' do