Set timezone config explicitly

To disable the following warning:
```
DEPRECATION WARNING: `to_time` will always preserve the receiver timezone rather than system local time in Rails 8.0. To opt in to the new behavior, set `config.active_support.to_time_preserves_timezone = :zone`. (called from <top (required)> at /Users/ryunosuke.sato/src/github.com/thoughtbot/ember-cli-rails/spec/spec_helper.rb:9)
```

spec/dummy/application.rb

@@ -33,6 +33,8 @@ class Application < Rails::Application
     config.secret_token = "SECRET_TOKEN_IS_MIN_30_CHARS_LONG"
     config.secret_key_base = "SECRET_KEY_BASE"
 
+    config.active_support.to_time_preserves_timezone = :zone
+
     def require_environment!
       initialize!
     end