Allow the child process to be already gone

Author: pitr-ch

spec/concurrent/executor/cached_thread_pool_spec.rb

@@ -212,6 +212,8 @@ module Concurrent
               pid       = spawn RbConfig.ruby, test_file
               Process.waitpid pid
               expect($?.success?).to eq true
+            rescue Errno::ECHILD
+              # child already gone
             rescue Timeout::Error => e
               Process.kill :KILL, pid
               raise e