Add finalize_background_migration spec matcher for testing

What does this MR do and why?

Adds a simple be_finalize_background_migration_of spec matcher to validate that finalize_background_migration was called correctly for the expected class.

How to set up and validate locally

1. Queue a background migration and add the finalize migration:

• db/post_migrate/20220426190417_schedule_test_background_migration.rb:

  # frozen_string_literal: true
  
  class ScheduleTestBackgroundMigration < Gitlab::Database::Migration[1.0]
    disable_ddl_transaction!
  
    MIGRATION = 'TestBackgroundMigration'
    DELAY_INTERVAL = 2.minutes
  
    def up
      queue_background_migration_jobs_by_range_at_intervals(
        define_batchable_model('users'),
        MIGRATION,
        DELAY_INTERVAL,
        track_jobs: true
      )
    end
  
    def down
      # no-op
    end
  end

• lib/gitlab/background_migration/test_background_migration.rb:

  # frozen_string_literal: true
  
  class Gitlab::BackgroundMigration::TestBackgroundMigration
    def perform(start_id, end_id)
      mark_job_as_succeeded(start_id, end_id)
    end
  
    private
  
    def mark_job_as_succeeded(*arguments)
      ::Gitlab::Database::BackgroundMigrationJob.mark_all_as_succeeded(
        self.class.name.demodulize,
        arguments
      )
    end
  end

• db/post_migrate/20220426193321_finalize_test_background_migration.rb:

  # frozen_string_literal: true
  
  class FinalizeTestBackgroundMigration < Gitlab::Database::Migration[1.0]
    disable_ddl_transaction!
  
    MIGRATION = 'TestBackgroundMigration'
  
    def up
      finalize_background_migration(MIGRATION)
    end
  
    def down
      # no-op
    end
  end

2. Add the spec using the matcher, spec/migrations/20220426193321_finalize_test_background_migration_spec.rb:

   # frozen_string_literal: true
   
   require 'spec_helper'
   require_migration!
   
   RSpec.describe FinalizeTestBackgroundMigration, :migration do
     let_it_be(:migration) { described_class::MIGRATION }
   
     describe '#up' do
       it 'finalizes the background migration' do
         expect(described_class).to be_finalize_background_migration_of(migration)
   
         migrate!
       end
     end
   end

3. bin/rails db:migrate RAILS_ENV=test and bin/rspec spec/migrations/20220426193321_finalize_test_background_migration_spec.rb, notice it passes

4. Change the MIGRATION name in the spec to something different, and see it failing, i.e. for expect(described_class).to be_finalize_background_migration_of('invalid'):

    expected: ("invalid")
    got: ("TestBackgroundMigration")
    # ./db/post_migrate/20220426193321_finalize_test_background_migration.rb:9:in `up'
    # ./lib/gitlab/database/migrations/lock_retry_mixin.rb:36:in `ddl_transaction'

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.