Generate deterministic fixture for users_select/index_spec

Description

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One contributing factor to this is that this fixture is very different in CE and EE

https://gitlab.com/gitlab-org/gitlab/blob/4735c99b1fdd7e413c288497eec9c45fb4ba9012/spec/frontend/fixtures/merge_requests.rb#L65

This is surprising to me. I thought licensed features were false by default in our spec environment 🤔

Investigation

Since there's some other tests using the old merge_request_for_current_user.html fixture, it's probably safest to try to add a brand new one just for that part of the view we are interested in.