Refactor `ee/spec/lib/remote_development/agent_config/updater_spec.rb`
The following discussion from !138732 should be addressed:
- [ ] @cwoolley-gitlab started a [discussion](https://gitlab.com/gitlab-org/gitlab/-/merge_requests/138732#note_1695788406):
> **suggestion (non-blocking)**: It's a bit confusing to have these booleans which control how the fixtures are constructed.
>
> I would like to revisit the design of this entire spec, and try to move towards a more declarative and idiomatic design for the fixtures, where we have minimal (ideally no) logic in the fixture definitions, and instead use different `let` and `let_it_be` declarations in different contexts to define the different scenarios.
>
> Also, since `let` and `let_it_be` are cached by default (as opposed to `let!` and `let_it_be(..., reload: true) { ... }`), so having conditional logic in their declarations can make the code hard to reason about.
>
> Here's our docs on best practices for using let: https://docs.gitlab.com/ee/development/testing_guide/best_practices.html#subject-and-let-variables
>
> But I don't want to block this MR on that, this has adequate coverage of all the cases, so it's fine for now.
>
> I'll make a follow-up issue to refactor it.
issue