title: Test an Apache Airflow DAG while it is already scheduled and running?
body: <p>I ran the following test command:</p>
  
  <pre><code>airflow test events {task_name_redacted} 2018-12-12
  </code></pre>
  
  <p>...and got the following output:</p>
  
  <pre><code>Dependencies not met for &lt;TaskInstance: events.{redacted} 2018-12-12T00:00:00+00:00 [None]&gt;, dependency 'Task Instance Slots Available' FAILED: The maximum number of running tasks (16) for this task's DAG 'events' has been reached.
  [2019-01-17 19:47:48,978] {models.py:1556} WARNING - 
  --------------------------------------------------------------------------------
  FIXME: Rescheduling due to concurrency limits reached at task runtime. Attempt 1 of 6. State set to NONE.
  --------------------------------------------------------------------------------
  
  [2019-01-17 19:47:48,978] {models.py:1559} INFO - Queuing into pool None
  </code></pre>
  
  <p>My Airflow is configured with a maximum concurrency of 16. Does this mean that I cannot test a task when the DAG is currently running, and has used all of it's task slots?</p>
  
  <p>Also, it was a little unclear from the docs, but does the <code>airflow test</code> actually execute the task, as in if it was a <code>SparkSubmitOperator</code>, it would actually submit the job?</p>