Don't depend on user model in migration spec

The User model now has a default value for a field that didn't exist when these
migrations ran.

spec/lib/gitlab/background_migration/migrate_events_to_push_event_payloads_spec.rb

@@ -215,9 +215,16 @@ end
 # to a specific version of the database where said table is still present.
 #
 describe Gitlab::BackgroundMigration::MigrateEventsToPushEventPayloads, :migration, schema: 20170825154015 do
+  let(:user_class) do
+    Class.new(ActiveRecord::Base) do
+      self.table_name = 'users'
+    end
+  end
+
   let(:migration) { described_class.new }
-  let(:project) { create(:project_empty_repo) }
-  let(:author) { create(:user) }
+  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) }
 
   # We can not rely on FactoryGirl as the state of Event may change in ways that
   # the background migration does not expect, hence we use the Event class of

spec/migrations/convert_custom_notification_settings_to_columns_spec.rb

@@ -2,6 +2,12 @@ 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(:settings_params) do
     [
       { level: 0, events: [:new_note] }, # disabled, single event
@@ -19,7 +25,7 @@ describe ConvertCustomNotificationSettingsToColumns, :migration do
         events[event] = true
       end
 
-      user = create(:user)
+      user = build(:user).becomes(user_class).tap(&:save!)
       create_params = { user_id: user.id, level: params[:level], events: events }
       notification_setting = described_class::NotificationSetting.create(create_params)
 
@@ -35,7 +41,7 @@ describe ConvertCustomNotificationSettingsToColumns, :migration do
         events[event] = true
       end
 
-      user = create(:user)
+      user = build(:user).becomes(user_class).tap(&:save!)
       create_params = events.merge(user_id: user.id, level: params[:level])
       notification_setting = described_class::NotificationSetting.create(create_params)