test(pubsublite): fix flaky TestAssigningSubscriberAddRemovePartitions (

#8496)

Wait for commit requests to reach the server before ending the test.

Fixes: https://togithub.com/googleapis/google-cloud-go/issues/8459

pubsublite/internal/wire/subscriber_test.go

@@ -1190,7 +1190,7 @@ func TestAssigningSubscriberAddRemovePartitions(t *testing.T) {
 	cmtStream3 := test.NewRPCVerifier(t)
 	cmtStream3.Push(initCommitReq(subscriptionPartition{Path: subscription, Partition: 3}), initCommitResp(), nil)
 	cmtStream3.Push(commitReq(34), commitResp(1), nil)
-	cmtStream3.Push(commitReq(35), commitResp(1), nil)
+	cmt2Barrier := cmtStream3.PushWithBarrier(commitReq(35), commitResp(1), nil)
 	verifiers.AddCommitStream(subscription, 3, cmtStream3)
 
 	// Partition 6
@@ -1203,7 +1203,7 @@ func TestAssigningSubscriberAddRemovePartitions(t *testing.T) {
 
 	cmtStream6 := test.NewRPCVerifier(t)
 	cmtStream6.Push(initCommitReq(subscriptionPartition{Path: subscription, Partition: 6}), initCommitResp(), nil)
-	cmtStream6.Push(commitReq(67), commitResp(1), nil)
+	cmt3Barrier := cmtStream6.PushWithBarrier(commitReq(67), commitResp(1), nil)
 	verifiers.AddCommitStream(subscription, 6, cmtStream6)
 
 	// Partition 8
@@ -1214,7 +1214,7 @@ func TestAssigningSubscriberAddRemovePartitions(t *testing.T) {
 
 	cmtStream8 := test.NewRPCVerifier(t)
 	cmtStream8.Push(initCommitReq(subscriptionPartition{Path: subscription, Partition: 8}), initCommitResp(), nil)
-	cmtStream8.Push(commitReq(89), commitResp(1), nil)
+	cmt5Barrier := cmtStream8.PushWithBarrier(commitReq(89), commitResp(1), nil)
 	verifiers.AddCommitStream(subscription, 8, cmtStream8)
 
 	mockServer.OnTestStart(verifiers)
@@ -1243,8 +1243,11 @@ func TestAssigningSubscriberAddRemovePartitions(t *testing.T) {
 	msg4Barrier.Release()
 	receiver.ValidateMsgs(partitionMsgs(3, msg2))
 
-	// Ensure the second assignment ack is received by the server to avoid test
-	// flakiness.
+	// Ensure requests are received by the server to avoid test flakiness.
+	sub.FlushCommits()
+	cmt2Barrier.Release()
+	cmt3Barrier.Release()
+	cmt5Barrier.Release()
 	assignmentBarrier2.Release()
 
 	// Stop should flush all commit cursors.