unstable test in the TestNormalSessionPool

[hawkingrei]

Bug Report

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1. Minimal reproduce step (Required)

[2022-07-19T04:52:57.582Z]     db_test.go:81: 
[2022-07-19T04:52:57.582Z]         	Error Trace:	db_test.go:81
[2022-07-19T04:52:57.582Z]         	Error:      	Received unexpected error:
[2022-07-19T04:52:57.582Z]         	            	listen tcp 0.0.0.0:4000: bind: address already in use
[2022-07-19T04:52:57.582Z]         	            	github.com/pingcap/errors.AddStack
[2022-07-19T04:52:57.582Z]         	            		external/com_github_pingcap_errors/errors.go:174
[2022-07-19T04:52:57.582Z]         	            	github.com/pingcap/errors.Trace
[2022-07-19T04:52:57.582Z]         	            		external/com_github_pingcap_errors/juju_adaptor.go:15
[2022-07-19T04:52:57.582Z]         	            	github.com/pingcap/tidb/server.NewServer
[2022-07-19T04:52:57.582Z]         	            		server/server.go:250
[2022-07-19T04:52:57.582Z]         	            	github.com/pingcap/tidb/domain_test.TestNormalSessionPool
[2022-07-19T04:52:57.582Z]         	            		domain/db_test.go:80
[2022-07-19T04:52:57.582Z]         	            	testing.tRunner
[2022-07-19T04:52:57.582Z]         	            		GOROOT/src/testing/testing.go:1439
[2022-07-19T04:52:57.582Z]         	            	runtime.goexit
[2022-07-19T04:52:57.582Z]         	            		GOROOT/src/runtime/asm_amd64.s:1571
[2022-07-19T04:52:57.582Z]         	Test:       	TestNormalSessionPool

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

3. What did you see instead (Required)

4. What is your TiDB version? (Required)