type.

This allows specific jobs to be tested, while preventing others from
being performed unnecessarily.

Example:

    def test_hello_job
      assert_performed_jobs 1, only: HelloJob do
        HelloJob.perform_later('jeremy')
        LoggingJob.perform_later
      end
    end

An array may also be specified, to support testing multiple jobs.

Example:

    def test_hello_and_logging_jobs
      assert_nothing_raised do
        assert_performed_jobs 2, only: [HelloJob, LoggingJob] do
          HelloJob.perform_later('jeremy')
          LoggingJob.perform_later('stewie')
          RescueJob.perform_later('david')
        end
      end
    end

Fixes #18802.

Trim space and document :only option.