unstable test in the TestTopSQLReporter

[hawkingrei]

Bug Report

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1. Minimal reproduce step (Required)

 === FAIL: util/topsql TestTopSQLReporter (1.42s)
[2022/02/18 04:12:57.950 +00:00] [INFO] [cpuprofile.go:115] ["parallel cpu profiler started"]
[2022/02/18 04:12:57.951 +00:00] [INFO] [cpu.go:84] ["sql cpu collector started"]
    topsql_test.go:168: 
        	Error Trace:	topsql_test.go:168
        	Error:      	Not equal: 
        	            	expected: 2
        	            	actual  : 1
        	Test:       	TestTopSQLReporter
[2022/02/18 04:12:59.234 +00:00] [INFO] [cpu.go:99] ["sql cpu collector stopped"]
[2022/02/18 04:12:59.368 +00:00] [INFO] [cpuprofile.go:132] ["parallel cpu profiler stopped"] 

https://prow.hawkingrei.com/view/gs/prowpingcap/logs/tidb_race/1494519538325131264

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

3. What did you see instead (Required)

4. What is your TiDB version? (Required)