flaky test in the TestExtensionConnEvent

[hawkingrei]

Bug Report

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1. Minimal reproduce step (Required)

    tidb_test.go:2893: 

        	Error Trace:	/home/jenkins/.tidb/tmp/63a9840cd0739f2c243bb46478607469/sandbox/linux-sandbox/7024/execroot/__main__/bazel-out/k8-fastbuild/bin/server/server_test_/server_test.runfiles/__main__/server/tidb_test.go:2893

        	            				/home/jenkins/.tidb/tmp/63a9840cd0739f2c243bb46478607469/sandbox/linux-sandbox/7024/execroot/__main__/bazel-out/k8-fastbuild/bin/server/server_test_/server_test.runfiles/__main__/server/tidb_test.go:2781

        	            				/home/jenkins/.tidb/tmp/63a9840cd0739f2c243bb46478607469/sandbox/linux-sandbox/7024/execroot/__main__/bazel-out/k8-fastbuild/bin/server/server_test_/server_test.runfiles/__main__/server/tidb_test.go:2892

        	Error:      	Not equal: 

        	            	expected: []extension.ConnEventTp{0x0, 0x2, 0x4}

        	            	actual  : []extension.ConnEventTp{0x0, 0x2}

        	            	

        	            	Diff:

        	            	--- Expected

        	            	+++ Actual

        	            	@@ -1,3 +1,3 @@

        	            	-([]extension.ConnEventTp) (len=3) {

        	            	- 00000000  00 02 04                                          |...|

        	            	+([]extension.ConnEventTp) (len=2) {

        	            	+ 00000000  00 02                                             |..|

        	            	 }

        	Test:       	TestExtensionConnEvent

https://do.pingcap.net/jenkins/blue/organizations/jenkins/pingcap%2Ftidb%2Fghpr_unit_test/detail/ghpr_unit_test/6485/pipeline

2. What did you expect to see? (Required)

3. What did you see instead (Required)

4. What is your TiDB version? (Required)

[lcwangchao]

/run-check-issue-triage-complete