Add Crystal::EventLoop#drain(&)

src/crystal/event_loop.cr

@@ -69,6 +69,13 @@ abstract class Crystal::EventLoop
     abstract def run(queue : Fiber::List*, blocking : Bool) : Nil
   {% end %}
 
+  # Blocks the current scheduler until all the pending events have completed.
+  # Must yield every runnable fiber.
+  #
+  # Optional.
+  def drain(& : Fiber ->) : Nil
+  end
+
   # Tells a blocking run loop to no longer wait for events to activate. It may
   # for example enqueue a NOOP event with an immediate (or past) timeout. Having
   # activated an event, the loop shall return, allowing the blocked thread to

src/fiber/execution_context/parallel/scheduler.cr

@@ -130,7 +130,10 @@ module Fiber::ExecutionContext
         loop do
           if @shutdown
             spin_stop
+
+            # drain everything into the global queue
             @runnables.drain
+            @event_loop.drain { |fiber| @global_queue.push(fiber) }
 
             # we may have been the last running scheduler, waiting on the event
             # loop while there are pending events for example; let's resume a