Fix BBM worker spec failures

Example failures:

• https://gitlab.com/gitlab-org/gitlab/-/jobs/11239470273
• https://gitlab.com/gitlab-org/gitlab/-/jobs/11239470346

These specs are failing with:

     # --- Caused by: ---
     # PG::UndefinedTable:
     #   ERROR:  relation "_test_example_data" does not exist
     #   LINE 10:  WHERE a.attrelid = '"_test_example_data"'::regclass
     #            

But this table is created in the spec here: https://gitlab.com/gitlab-org/gitlab/-/blob/26b6b86415b78e31c9d52ac63563ef546daf0b09/spec/support/shared_examples/workers/batched_background_migration_worker_shared_examples.rb#L278-291