It's very easy to forget to add `require 'rails_helper'` at the top of every core/plugin spec file, and omissions can cause some very confusing/sporadic errors.
By setting this flag in `.rspec`, we can remove the need for `require 'rails_helper'` entirely.
This only affects multisite Discourse instances (where multiple forums are served from a single application server). The vast majority of self-hosted Discourse forums do not fall into this category.
On affected instances, this vulnerability could allow encrypted session cookies to be re-used between sites served by the same application instance.
