Avoid race condition in puma plugin test between restart and termination

If the restart is too slow, we might process the jobs with the initial run of
puma instead of the restarted process, and then when we try to terminate it
in teardown, we fail.

Author: rosa

test/integration/puma/plugin_test.rb

@@ -41,7 +41,8 @@ class PumaPluginTest < ActiveSupport::TestCase
 
   test "stop the queue on puma's restart" do
     signal_process(@pid, :SIGUSR2)
-
+    # Ensure the restart finishes before we try to continue with the test
+    wait_for_registered_processes(0, timeout: 3.second)
     wait_for_registered_processes(4, timeout: 3.second)
 
     StoreResultJob.perform_later(:puma_plugin)