fix: flaky SubscriptionTest (#3437)

Co-authored-by: Andrew (Paradi) Alexander <[email protected]>

Author: jvmakine

misk-aws2-sqs/src/test/kotlin/misk/aws2/sqs/jobqueue/ExampleHandler.kt

@@ -7,16 +7,17 @@ import misk.jobqueue.v2.JobStatus
 import misk.jobqueue.v2.SuspendingJobHandler
 import misk.logging.getLogger
 import java.util.concurrent.CountDownLatch
+import java.util.concurrent.CopyOnWriteArrayList
 
 @Singleton
 class ExampleHandler @Inject constructor(): SuspendingJobHandler {
   internal val counter = CountDownLatch(3)
-  internal val jobs = mutableListOf<Job>()
+  internal val jobs = CopyOnWriteArrayList<Job>()
 
   override suspend fun handleJob(job: Job): JobStatus {
     logger.info { "Handling job $job, current counter $counter" }
-    counter.countDown()
-    jobs.add(job)
+    jobs.add(job)        // Add job to list first
+    counter.countDown()  // Then signal completion
     return JobStatus.OK
   }