Review assert_no_emails

[gravitystorm]

We've started using ActionMailer::TestHelper, i.e.

# NB: loads helpers to verify mailer-related behaviour e.g. via assert_no_emails
  include ActionMailer::TestHelper

We should do this consistently throughout the test suite. Before making the change though, we need to double-check if it's assert_no_email or assert_no_enqueued_emails that we should be using.