Run tests with Rails default configuration to enable Zeitwerk

[bensheldon]

GoodJob requires Rails use the Zeitwerk autoloader (not Classic) for Rails 6.0+.

This PR ensures that that the test environment defaults to the configuration defaults for the version of Rails the test suit is being run against.

This was discovered because of a deadlock caused by isolating a single test that loads up a bunch of Scheduler threads at once. I think it's the same deadlock that causes CI tests to flakily time-out:

$ RBTRACE=1 bin/rspec spec/integration/scheduler_spec.rb:52

good_job/spec/integration/scheduler_spec.rb

Lines 52 to 57 in bed770d

	it 'pops items off of the queue and runs them' do
	max_threads = 5
	performer = GoodJob::Performer.new(GoodJob::Job.all, :perform_with_advisory_lock)
	scheduler = GoodJob::Scheduler.new(performer, max_threads: max_threads)
	max_threads.times { scheduler.create_thread }