Use rake_helper for backup.rake spec

This helper is being used implicitly on CI where all the tests are
running together. So to make sure we get consistent results in dev we
need to include it explicitly.

spec/tasks/gitlab/backup_rake_spec.rb

 # frozen_string_literal: true
 
-require 'spec_helper'
-require 'rake'
+require 'rake_helper'
 
 RSpec.describe 'gitlab:app namespace rake task', :delete do
   let(:enable_registry) { true }
@@ -24,14 +23,10 @@ RSpec.describe 'gitlab:app namespace rake task', :delete do
 
   before(:all) do
     Rake.application.rake_require 'active_record/railties/databases'
-    Rake.application.rake_require 'tasks/gitlab/helpers'
     Rake.application.rake_require 'tasks/gitlab/backup'
     Rake.application.rake_require 'tasks/gitlab/shell'
     Rake.application.rake_require 'tasks/gitlab/db'
     Rake.application.rake_require 'tasks/cache'
-
-    # empty task as env is already loaded
-    Rake::Task.define_task :environment
   end
 
   before do
@@ -39,6 +34,7 @@ RSpec.describe 'gitlab:app namespace rake task', :delete do
     FileUtils.rm(tars_glob, force: true)
     FileUtils.rm(backup_files, force: true)
     FileUtils.rm_rf(backup_directories, secure: true)
+    FileUtils.mkdir_p('tmp/tests/public/uploads')
     reenable_backup_sub_tasks
     stub_container_registry_config(enabled: enable_registry)
   end
@@ -47,12 +43,7 @@ RSpec.describe 'gitlab:app namespace rake task', :delete do
     FileUtils.rm(tars_glob, force: true)
     FileUtils.rm(backup_files, force: true)
     FileUtils.rm_rf(backup_directories, secure: true)
-  end
-
-  def run_rake_task(task_name)
-    FileUtils.mkdir_p('tmp/tests/public/uploads')
-    Rake::Task[task_name].reenable
-    Rake.application.invoke_task task_name
+    FileUtils.rm_rf('tmp/tests/public/uploads', secure: true)
   end
 
   def reenable_backup_sub_tasks