CI unstable test

[tiancaiamao]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. Minimal reproduce step (Required)

I don't know how it happen, found in https://ci.pingcap.net/blue/organizations/jenkins/tidb_ghpr_unit_test/detail/tidb_ghpr_unit_test/24143/pipeline

[2021-12-10T08:08:23.549Z] --- FAIL: TestExprPushDownToFlash (0.01s)
[2021-12-10T08:08:23.549Z]     expr_to_pb_test.go:915:
[2021-12-10T08:08:23.549Z]              Error Trace:    expr_to_pb_test.go:915
[2021-12-10T08:08:23.549Z]              Error:          Not equal:
[2021-12-10T08:08:23.549Z]                              expected: true
[2021-12-10T08:08:23.549Z]                              actual  : false
[2021-12-10T08:08:23.549Z]              Test:           TestExprPushDownToFlash

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

CI pass

3. What did you see instead (Required)

CI fail

4. What is your TiDB version? (Required)

#30066

[jebter]

The issue has not been updated for too long, so I will close it. If there are any updates, you can reopen it.