Extend tests to make sure all required refs are kept after a project import
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Currently, the specs for a project import in https://gitlab.com/gitlab-org/gitlab/-/blob/d9a3bad64c6e2b281acdb6f5e3370ef00b705dcf/spec/services/projects/after_import_service_spec.rb#L46 iterate the `Repository::RESERVED_REFS_NAMES` to check if refs are kept. But this does not verify if all expected refs are actually in the constant. Which caused a bug in the past.
The following discussion from !82326 should be addressed:
- [ ] @reprazent started a [discussion](https://gitlab.com/gitlab-org/gitlab/-/merge_requests/82326#note_867075675): (+1 comment)
> `@exit404` Thanks for tackling this. And for including the history in the description :heart:. I think this makes sense. But I do wonder if we should improve the specs to prevent this from happening again, because we [iterate](https://gitlab.com/gitlab-org/gitlab/-/blob/d9a3bad64c6e2b281acdb6f5e3370ef00b705dcf/spec/services/projects/after_import_service_spec.rb#L46) the constant that we're also testing, this why it went unnoticed before.
>
> `@wortschi` What do you think? I don't think this should block merging this though, this is already solving a problem.
issue