Cable: Shush pop warnings when skipping Postgres tests.

`skip` raises an exception to abort the execution of the test, so
`super` would never be called and thus `@rx_adapter` and `@tx_adapter`
would never have been defined at the time of teardown.

Define them just before skipping and zap the warnings.

actioncable/test/subscription_adapter/postgresql_test.rb

@@ -21,6 +21,7 @@ def setup
     begin
       ActiveRecord::Base.connection
     rescue
+      @rx_adapter = @tx_adapter = nil
       skip "Couldn't connect to PostgreSQL: #{database_config.inspect}"
     end