Add spec for background migration

spec/lib/gitlab/background_migration/backfill_incident_issue_escalation_statuses_spec.rb

+# frozen_string_literal: true
+
+require 'spec_helper'
+
+RSpec.describe Gitlab::BackgroundMigration::BackfillIncidentIssueEscalationStatuses do
+  let(:namespaces)                    { table(:namespaces) }
+  let(:projects)                      { table(:projects) }
+  let(:issues)                        { table(:issues) }
+  let(:issuable_escalation_statuses)  { table(:incident_management_issuable_escalation_statuses) }
+
+  subject(:migration) { described_class.new }
+
+  it 'correctly backfills issuable escalation status records' do
+    namespace = namespaces.create!(name: 'foo', path: 'foo')
+    project = projects.create!(namespace_id: namespace.id)
+
+    regular_issue    = issues.create!(project_id: project.id, title: 'issue 1', issue_type: 0)
+    incident_issue_1 = issues.create!(project_id: project.id, title: 'incident 1', issue_type: 1)
+    incident_issue_2 = issues.create!(project_id: project.id, title: 'incident 2', issue_type: 1)
+    incident_issue_existing_status = issues.create!(project_id: project.id, title: 'incident 3', issue_type: 1)
+    existing_status = issuable_escalation_statuses.create!(issue_id: incident_issue_existing_status.id)
+
+    migration.perform(1, incident_issue_existing_status.id)
+
+    expect(issuable_escalation_statuses.count).to eq(3)
+  end
+end