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
SiteSettingExtension triggers message bus which re-establishes a
DB connection in `SiteSettingExtension#process_message`. That happens
concurrently and a test that requires a connection to the db will
fail when the reconnection is happening.