Use Rails environment in all tests.

Too much work to keep them isolated given global I18n config.
Fall back to default locale on unknown subdomain.

This allows using subdomains for quick testing on non-official domain.
For example, testing mobile site with xip.io where locale-subdomain would not be correctly identified.
This shouldn't affect production websites since you already explicitly whitelist allowed subdomains.
For any long-term domain you should adjust config.action_dispatch.tld_length to make locale links work.
E.g. beta.example.com → tld_length = 2
Add config setting for default subdomain. No subdomain is now standard.