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:
  job_data = job_to_hash(job)
  perform_or_enqueue(perform_enqueued_jobs && !filtered?(job), job, job_data)
end

def enqueue_at(job, timestamp) # :nodoc:

:nodoc: 
def enqueue_at(job, timestamp) # :nodoc:
  job_data = job_to_hash(job, at: timestamp)
  perform_or_enqueue(perform_enqueued_at_jobs && !filtered?(job), job, job_data)
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 filter_as_proc(filter) 

def filter_as_proc(filter)
  return filter if filter.is_a?(Proc)
  ->(job) { Array(filter).include?(job.class) }
end

def filtered?(job) 

def filtered?(job)
  filtered_queue?(job) || filtered_job_class?(job) || filtered_time?(job)
end

def filtered_job_class?(job) 

def filtered_job_class?(job)
  if filter
    !filter_as_proc(filter).call(job)
  elsif reject
    filter_as_proc(reject).call(job)
  end
end

def filtered_queue?(job) 

def filtered_queue?(job)
  if queue
    job.queue_name != queue.to_s
  end
end

def filtered_time?(job) 

def filtered_time?(job)
  job.scheduled_at > at.to_f if at && job.scheduled_at
end

def job_to_hash(job, extras = {}) 

def job_to_hash(job, extras = {})
  job.serialize.tap do |job_data|
    job_data[:job] = job.class
    job_data[:args] = job_data.fetch("arguments")
    job_data[:queue] = job_data.fetch("queue_name")
    job_data[:priority] = job_data.fetch("priority")
  end.merge(extras)
end

def perform_or_enqueue(perform, job, job_data) 

def perform_or_enqueue(perform, job, job_data)
  if perform
    performed_jobs << job_data
    Base.execute(job.serialize)
  else
    enqueued_jobs << job_data
  end
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