Follow-up from "Migrate merge requests mentions to DB table"

Summary

spec/lib/gitlab/background_migration/user_mentions/create_resource_user_mention_spec.rb spec is dependent on current app structure

The following discussion from !25826 (merged) should be addressed:

• @pshutsin started a discussion:

  @acroitor @toon @smcgivern Using becomes(User) and becomes(Namespace) makes migration spec dependent on app code and current DB structure.

  See example failed pipeline where user_type column was introduced, There is no user_type column for this migration spec but User class requires it.

  I'll short-fix the spec for my local branch, and will create an issue to fix it properly.

Steps to reproduce

1. Add new column to Users table. E.g. add_column :users, :my_new_attr, null: false
2. Add Rails-validation to new column validates :my_new_attr, presence: true
3. Run bundle exec rspec spec/lib/gitlab/background_migration/user_mentions/create_resource_user_mention_spec.rb
4. => Spec will fail because User class will demand my_new_attr to be present, but there will be no my_new_attr because the spec is run against specific schema version schema: 20200211155539

Implications

This spec has a change to fail every time there is a database OR validation changes to User and Namespace models.

@acroitor could you lead the fix please?