These specs were disabled in 786f7503. While investigating this, I found out that at some point `:user_membership` got deleted. It's hard to tell why exactly without investing more time, but it seems using `let!` instead of `fab!` solves the issue.
If in the future we decide to investigate why these tests were flaky with `fab!` to reproduce the failure run:
LOAD_PLUGINS=1 rspec --seed 46586 plugins/chat/spec/mailers/user_notifications_spec.rb
6491be9f2b