This change both speeds up specs (less strings to allocate) and helps catch
cases where methods in Discourse are mutating inputs.
Overall we will be migrating everything to use #frozen_string_literal: true
it will take a while, but this is the first and safest move in this direction
This commit introduces a new helper to enable transactional fixtures
when testing multisite. This would show up as tests that passed the
first time then failed the second time due to stale data being leftover.
