flaky test TestConcurrentLock

[hawkingrei]

Bug Report

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3. What did you see instead (Required)

 === RUN   TestConcurrentLock
    locking_test.go:111: 
        	Error Trace:	br/pkg/storage/locking_test.go:111
        	Error:      	Received unexpected error:
        	            	there is conflict file test.lock.INTENT.b310aa1ae9a9453392771608458f4ecc
        	            	during checking whether there are other intentions
        	            	github.com/pingcap/tidb/br/pkg/storage.conditionalPut.CommitTo
        	            		br/pkg/storage/locking.go:98
        	            	github.com/pingcap/tidb/br/pkg/storage.TryLockRemote
        	            		br/pkg/storage/locking.go:217
        	            	github.com/pingcap/tidb/br/pkg/storage_test.TestConcurrentLock.func2
        	            		br/pkg/storage/locking_test.go:98
        	            	runtime.goexit
        	            		src/runtime/asm_amd64.s:1700
        	            	there is something about the lock: failed to read existed lock file test.lock: open /tmp/TestConcurrentLock3301147168/001/test.lock: no such file or directory
        	Test:       	TestConcurrentLock
--- FAIL: TestConcurrentLock (0.00s) 

https://tiprow.hawkingrei.com/view/gs/pingcapprow/pr-logs/pull/pingcap_tidb/57718/fast_test_tiprow/1861643693912494080#1:build-log.txt%3A1923

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