Ensure emails in tests go via Sidekiq

https://github.com/rails/rails/issues/37270#issuecomment-553927324 has popped up a couple of times, most recently in !62814 (merged).

This MR sidesteps it by ensuring that we don't have this weird dichotomy for our test jobs: instead of emails going through the ActiveJob test executor, and Sidekiq jobs going through the Sidekiq one, we make everything go via Sidekiq.