Use Rails' transactional fixtures instead of database cleaner

[bmulholland]

This builds on #26, I suppose

Rails has built-in transactional fixtures, which should mean database cleaner is never needed: https://anti-pattern.com/transactional-fixtures-in-rails

As far as I can tell, this just means including ActiveRecord::TestFixtures, then calling setup_fixtures and teardown_fixtures. See https://github.com/rails/rails/blob/812bae8167488ee5984043ef71125e055352b873/activerecord/lib/active_record/test_fixtures.rb

[bmulholland]

I tried but ran into issues with threading. Specifically, I'd get errors like Read: #<ActiveRecord::ActiveRecordError: Cannot expire connection, it is owned by a different thread: #<Thread:0x0000000111657040@puma srv tp 004 and #<ActiveRecord::ConnectionTimeoutError: could not obtain a connection from the pool within 5.000 seconds (waited 5.003 seconds); all pooled connections were in use>

Here's what I tried writing in clean.rb. I refered to https://github.com/rails/rails/blob/812bae8167488ee5984043ef71125e055352b873/activerecord/lib/active_record/test_fixtures.rb and https://github.com/rspec/rspec-rails/blob/main/lib/rspec/rails/fixture_support.rb

class Fixtures
  include ActiveRecord::TestFixtures
  extend ActiveSupport::Concern

  def run_in_transaction?
    true
  end

  included do
    self.use_transactional_tests = true
    self.use_instantiated_fixtures = false
  end
end

@@fixtures ||= Fixtures.new
# teardown is done *before* each step to follow Cypress best practices of
# tearing down state only in *before* hooks:
# https://docs.cypress.io/guides/references/best-practices#Using-after-or-afterEach-hooks
if @@fixtures.respond_to?(:@fixture_connections, true) &&
   @@fixtures.send(:@fixture_connections).any?
  @@fixtures.teardown_fixtures
end
@@fixtures.setup_fixtures