Write a spec covering the race condition during group deletion.

- Use multiple threads / database connections to:

  1. Escape the transaction the spec seems to be running
     in (`config.use_transactional_fixtures` is off, but
     `ActiveRecord::Base.connection.open_transactions` is not empty
     at the beginning of the spec.

  2. Simulate a Sidekiq worker performing the hard delete outside of the
     soft-delete transaction.

- The spec is a little clunky, but it was the smallest thing I could get
  working - and even this took a couple of hours. Let me know if you
  have any suggestions to improve it!