Work around migration specs failing due to repository storages not being selected

Migration specs may be missing a column necessary to create an ApplicationSetting
object, which causes the FakeApplicationSetting to be initialized. However, the
project selects a random repository storage via ApplicationSetting#pick_repository_storage,
which isn't present in the OpenStruct.

To workaround this issues, we add a pick_repository_storage method in
FakeApplicationSetting.

lib/gitlab/fake_application_settings.rb

@@ -11,6 +11,10 @@ module Gitlab
       FakeApplicationSettings.define_predicate_methods(options)
     end
 
+    def pick_repository_storage
+      repository_storages.sample
+    end
+
     # Mimic ActiveRecord predicate methods for boolean values
     def self.define_predicate_methods(options)
       options.each do |key, value|