Don't handle this change for legacy_connection_handling

PR #45773 broke legacy handling test fixtures and while I can fix this particular
error it breaks how shared pools are setup. I don't think this is the same kind
of buggy on legacy handling so let's leave that behavior and only change
the new handling.

Author: eileencodes

activerecord/lib/active_record/test_fixtures.rb

@@ -137,6 +137,7 @@ def setup_fixtures(config = ActiveRecord::Base)
         @connection_subscriber = ActiveSupport::Notifications.subscribe("!connection.active_record") do |_, _, _, _, payload|
           spec_name = payload[:spec_name] if payload.key?(:spec_name)
           shard = payload[:shard] if payload.key?(:shard)
+          setup_shared_connection_pool if ActiveRecord.legacy_connection_handling
 
           if spec_name
             begin
@@ -146,7 +147,7 @@ def setup_fixtures(config = ActiveRecord::Base)
             end
 
             if connection
-              setup_shared_connection_pool
+              setup_shared_connection_pool unless ActiveRecord.legacy_connection_handling
 
               if !@fixture_connections.include?(connection)
                 connection.begin_transaction joinable: false, _lazy: false