DEV: Don't user before(:all)/after(:all) (#13389)

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`)

spec/models/theme_field_spec.rb

@@ -4,14 +4,10 @@
 require 'rails_helper'
 
 describe ThemeField do
-  after(:all) do
+  after do
     ThemeField.destroy_all
   end
 
-  before do
-    I18n.locale = :en
-  end
-
   describe "scope: find_by_theme_ids" do
     it "returns result in the specified order" do
       theme = Fabricate(:theme)