Rake::TaskManager.class_eval do
  def delete_task(task_name)
    @tasks.delete(task_name.to_s)
  end
  Rake.application.delete_task("db:test:purge")
  Rake.application.delete_task("db:test:prepare")
end

namespace :db do
  namespace :test do
    task :purge do
    end
    task :prepare do
    end
  end
end