Fix order-dependent ActionCable spec failure

What does this MR do and why?

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 #418937 (closed)

How to set up and validate locally

Run:

bundle exec rspec spec/graphql/resolvers/users/participants_resolver_spec.rb spec/initializers/action_cable_subscription_adapter_identifier_spec.rb

On master, this fails with something like:

Failures:

  1) ActionCableSubscriptionAdapterIdentifier override #identifier when id key is nil on cable.yml does not override server config id with action cable pid
     Failure/Error: expect(sub.connection[:id]).to eq('unix:///home/localuser/redis/redis.socket/0')

       expected: "unix:///home/localuser/redis/redis.socket/0"
            got: "ActionCable-PID-54546"

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