Use `table` helper in migration specs

spec/lib/gitlab/background_migration/migrate_events_to_push_event_payloads_spec.rb

@@ -222,6 +222,7 @@ describe Gitlab::BackgroundMigration::MigrateEventsToPushEventPayloads, :migrati
   end
 
   let(:migration) { described_class.new }
+  let(:user_class) { table(:users) }
   let(:author) { build(:user).becomes(user_class).tap(&:save!).becomes(User) }
   let(:namespace) { create(:namespace, owner: author) }
   let(:project) { create(:project_empty_repo, namespace: namespace, creator: author) }

spec/migrations/convert_custom_notification_settings_to_columns_spec.rb

@@ -2,11 +2,7 @@ require 'spec_helper'
 require Rails.root.join('db', 'post_migrate', '20170607121233_convert_custom_notification_settings_to_columns')
 
 describe ConvertCustomNotificationSettingsToColumns, :migration do
-  let(:user_class) do
-    Class.new(ActiveRecord::Base) do
-      self.table_name = 'users'
-    end
-  end
+  let(:user_class) { table(:users) }
 
   let(:settings_params) do
     [