Run tests with specified locale

We changed .env.test a while ago, but I just discovered it hadn't taken any effect because it was overwritten here. Now it's loaded from env var.

spec/base_spec_helper.rb

@@ -150,7 +150,8 @@ RSpec.configure do |config|
 
   # Reset locale for all specs.
   config.around(:each) do |example|
-    I18n.with_locale(:en_AU) { example.run }
+    locale = ENV.fetch('LOCALE', 'en_TST')
+    I18n.with_locale(locale) { example.run }
   end
 
   # Reset all feature toggles to prevent leaking.