Fix order-dependent spec

What does this MR do?

Use existing connection's db_config instead of Gitlab::Database.main.config.

This returns a ActiveRecord::DatabaseConfigurations::HashConfig which contains the db name.

How to setup and validate locally (strongly suggested)

bin/rspec './spec/lib/gitlab/database/connection_spec.rb[1:10:2:1]' './spec/lib/gitlab/sidekiq_logging/structured_logger_spec.rb[1:1:5:2:1,1:1:5:2:2]'

Related to #339969 (closed)