Fix transient error in merge_request_spec

What does this MR do and why?

A transient error in the merge_request_spec.rb was happening

  20) MergeRequest state machine transitions #mark_as_unmergeable when the status is cannot_be_merged_rechecking behaves like transition triggering mergeRequestMergeStatusUpdated GraphQL subscription when transaction is not committed behaves like transition not triggering mergeRequestMergeStatusUpdated GraphQL subscription is expected not to receive merge_request_merge_status_updated(*(any args)) 0 times
      Failure/Error: raise CrossDatabaseModificationAcrossUnsupportedTablesError, messages.join("\n\n")
      Gitlab::Database::QueryAnalyzers::PreventCrossDatabaseModification::CrossDatabaseModificationAcrossUnsupportedTablesError:
        Cross-database data modification of 'gitlab_main_cell, gitlab_ci' were detected within a transaction modifying the 'namespaces, project_ci_cd_settings, container_expiration_policies, project_pages_metadata, project_features, project_statistics, members, notification_settings, ci_project_mirrors' tables. 

Stan surmised

but it seems like for some reason the subject might not be created until inside the transaction?

This MR implements that fix, which seems to fix the problem.

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