flaky test TestConnectThrough636

[hawkingrei]

Bug Report

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3. What did you see instead (Required)

 === RUN   TestConnectThrough636
    ldap_common_test.go:68: 
        	Error Trace:	pkg/privilege/privileges/ldap/ldap_common_test.go:68
        	            				src/runtime/asm_amd64.s:1650
        	Error:      	Received unexpected error:
        	            	listen tcp 127.0.0.1:12186: bind: address already in use
        	Test:       	TestConnectThrough636
    ldap_common_test.go:109: 
        	Error Trace:	pkg/privilege/privileges/ldap/ldap_common_test.go:109
        	Error:      	Received unexpected error:
        	            	LDAP Result Code 200 "Network Error": dial tcp [::1]:12186: connect: connection refused
        	            	create ldap connection
        	            	github.com/pingcap/tidb/pkg/privilege/privileges/ldap.(*ldapAuthImpl).connectionFactory
        	            		pkg/privilege/privileges/ldap/ldap_common.go:184
        	            	github.com/pingcap/tidb/pkg/privilege/privileges/ldap.TestConnectThrough636
        	            		pkg/privilege/privileges/ldap/ldap_common_test.go:108
        	            	testing.tRunner
        	            		GOROOT/src/testing/testing.go:1595
        	            	runtime.goexit
        	            		src/runtime/asm_amd64.s:1650
        	Test:       	TestConnectThrough636
--- FAIL: TestConnectThrough636 (1.94s) 

https://tiprow.hawkingrei.com/view/gs/pingcapprow/pr-logs/pull/pingcap_tidb/56194/fast_test_tiprow/1838871558488068096#1:build-log.txt%3A1847

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