Make policy stop test deterministic. (#70)

Author: samuel-williams-shopify

test/async/container/policy.rb

@@ -254,14 +254,26 @@ def child_exit(container, child, status, name:, key:, **options)
 			end.new
 
 			container = Async::Container.best_container_class.new(policy: stop_policy)
+			trigger = IO.pipe
 
 			3.times do |i|
 				container.spawn(name: "worker-#{i}") do |instance|
 					instance.ready!
-					exit(1)
+					
+					# Only the first worker exits, and only once all workers are ready, so exactly one `child_exit` drives the stop. Otherwise multiple simultaneous exits can race the `stopping?` check and inflate `stop_count`.
+					if i.zero?
+						trigger.first.gets
+						exit(1)
+					else
+						sleep
+					end
 				end
 			end
 
+			container.wait_until_ready
+			
+			trigger.last.puts("exit")
+			
 			container.wait
 
 			expect(stop_policy.stop_count).to be == 1