Leaking state and non-obvious order (before :all runs *before* RailsHelper.test_setup) are not worth it.
A replacement PR for #13370. Fixes some flaky specs, e.g.
```
bin/rspec './spec/components/freedom_patches/translate_accelerator_spec.rb[1:3]' './spec/jobs/clean_up_user_export_topics_spec.rb[1:1]' --tag ~type:multisite --seed 35994
```
Also included:
* DEV: No need for locale reset (we do it anyway in rails_helper in `test_setup`)
This change both speeds up specs (less strings to allocate) and helps catch
cases where methods in Discourse are mutating inputs.
Overall we will be migrating everything to use #frozen_string_literal: true
it will take a while, but this is the first and safest move in this direction
