Stabilize uncooperative child fixture

Assisted-By: devx/a01026a8-dd63-4b91-bae4-aec7ee8f7fa3

Author: samuel-williams-shopify

fixtures/async/container/a_child.rb

@@ -35,10 +35,15 @@ def start_uncooperative_child
 				ready = ::IO.pipe
 
 				child = subject.call(name: "test-child") do
-					ready.last.puts "ready"
+					ready_sent = false
 
 					loop do
 						begin
+							unless ready_sent
+								ready.last.puts "ready"
+								ready_sent = true
+							end
+							
 							sleep
 						rescue Interrupt
 							# Ignore graceful shutdown.