BitBucketServer: Add missing specs

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When Introduce parallelised BitBucket Server Importer (!120931 - merged) was merged, the undercoverage job revealed some test gaps.

We should address the following:

• object_importer
  • import method
  • importer_class method
• advance_stage_worker
  • find_import_state method
  • next_stage_worker method
• pull_request_notes_importer
  • create_diff_note method
  • create_fallback_diff_note method
• pull_request_notes_importer
  • create_diff_note method
• loggable.rb
  • log_warn method
  • log_error method
• parallel_scheduling.rb
  • id_for_already_processed_cache method
  • sidekiq_worker_class method
  • collection_method method