class ActiveJob::QueueAdapters::TestAdapter

Rails.application.config.active_job.queue_adapter = :test
To use the test adapter set queue_adapter config to :test.
it makes a great tool to test your Rails application.
ActiveJob::TestCase and ActiveJob::TestHelper
The test adapter should be used only in testing. Along with
== Test adapter for Active Job

def enqueue(job) #:nodoc:

:nodoc: 
def enqueue(job) #:nodoc:
  if perform_enqueued_jobs
    performed_jobs << {job: job.class, args: job.arguments, queue: job.queue_name}
    job.perform_now
  else
    enqueued_jobs << {job: job.class, args: job.arguments, queue: job.queue_name}
  end
end

def enqueue_at(job, timestamp) #:nodoc:

:nodoc: 
def enqueue_at(job, timestamp) #:nodoc:
  if perform_enqueued_at_jobs
    performed_jobs << {job: job.class, args: job.arguments, queue: job.queue_name, at: timestamp}
    job.perform_now
  else
    enqueued_jobs << {job: job.class, args: job.arguments, queue: job.queue_name, at: timestamp}
  end
end

def enqueued_jobs

Provides a store of all the enqueued jobs with the TestAdapter so you can check them. 
def enqueued_jobs
  @enqueued_jobs ||= []
end

def performed_jobs

Provides a store of all the performed jobs with the TestAdapter so you can check them. 
def performed_jobs
  @performed_jobs ||= []
end