Fix loading of take tasks in backup_rake_spec

Loading all rake tasks affects other specs
(specifically web_hook_rake_spec.rb) when run
before them. This restores the previous rake_require
and adds on "active_record/railties/databases" task
which defines "db:structure:dump" task.

spec/tasks/gitlab/backup_rake_spec.rb

@@ -23,7 +23,12 @@ describe 'gitlab:app namespace rake task' do
   end
 
   before(:all) do
-    Rails.application.load_tasks
+    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