Traceflow e2e test failed in DualStack Cluster

[gran-vmv]

Describe the bug
Traceflow e2e test failed in DualStack Cluster

To Reproduce
Run "/test-ipv6-ds-e2e" at any PR.
[OR]
Setup a DualStack e2e test Cluster.
Run "go test -v -timeout=30m github.com/vmware-tanzu/antrea/test/e2e -run TestTraceflowIntraNode"

Expected
The result should be "PASS".

Actual behavior
Some test case may fail like:
--- FAIL: TestTraceflowIntraNode/traceflowGroupTest/intraNodeUDPDstIPTraceflowIPv6 (15.02s)
[OR]
--- FAIL: TestTraceflowIntraNode/traceflowGroupTest/intraNodeICMPDstIPTraceflowIPv6 (15.02s)

Versions:
Please provide the following information:

• Antrea version (Docker image tag).
  Antrea main branch at 2021/4/21
• Kubernetes version (use kubectl version). If your Kubernetes components have different versions, please provide the version for all of them.
  K8s 1.19
• Container runtime: which runtime are you using (e.g. containerd, cri-o, docker) and which version are you using?
  docker community 19.03.12
• Linux kernel version on the Kubernetes Nodes (uname -r).
  4.15.0-142-generic
• If you chose to compile the Open vSwitch kernel module manually instead of using the kernel module built into the Linux kernel, which version of the OVS kernel module are you using? Include the output of modinfo openvswitch for the Kubernetes Nodes.
  vermagic: 4.15.0-142-generic SMP mod_unload modversions

Additional context
According to the flow table, the traceflow packet was correctly captured and was sent to controller (antrea-agent).
cookie=0x5000000000000, duration=9.169s, table=110, n_packets=1, n_bytes=62, hard_timeout=20, priority=202,ipv6,reg0=0x10000/0x10000,nw_tos=28 actions=controller(max_len=128,id=36780)
However, the antrea-agent didn't receive the PacketIn message. Below log should be reported when PacketIn message received, but it doesn't exist for the failed case.
I0421 09:05:39.722783 1 ofctrl_bridge.go:210] Received packet: &{Header:{Version:4 Type:10 Length:240 Xid:0} BufferId: ...

[gran-vmv]

According to local test, this is because installed OVS flows cannot take effect immediately.
Will add a small sleep to fix this.