Unstable test TestPlanStatsLoad

[zimulala]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. Minimal reproduce step (Required)

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

pass

3. What did you see instead (Required)

    plan_stats_test.go:110:
        	Error Trace:	planner/core/plan_stats_test.go:110
        	            				planner/core/plan_stats_test.go:216
        	Error:      	"0" is not greater than "0"
        	Test:       	TestPlanStatsLoad
[2023/03/22 10:02:12.687 +08:00] [INFO] [ddl_workerpool.go:82] ["[ddl] closing workerPool"]

4. What is your TiDB version? (Required)

Reproduce in #42454
https://do.pingcap.net/jenkins/blue/organizations/jenkins/pingcap%2Ftidb%2Fghpr_unit_test/detail/ghpr_unit_test/20200/pipeline