Warns if the BBM gets finalized earlier than expected

...ound_migration/batched_background_migration_dictionary.rb

...tabase/migrations/batched_background_migration_helpers.rb

lib/gitlab/utils/batched_background_migrations_dictionary.rb

rubocop/batched_background_migrations_dictionary.rb

rubocop/cop/background_migration/dictionary_file.rb

rubocop/cop/migration/unfinished_dependencies.rb

...migration/batched_background_migration_dictionary_spec.rb

...e/migrations/batched_background_migration_helpers_spec.rb

...ab/utils/batched_background_migrations_dictionary_spec.rb

.../rubocop/cop/background_migration/dictionary_file_spec.rb

spec/rubocop/cop/migration/unfinished_dependencies_spec.rb

spec/support/helpers/migrations_helpers.rb

spec/support_specs/helpers/migrations_helpers_spec.rb