unstable test in the TestCheckActRowsWithUnistore

## Bug Report

Please answer these questions before submitting your issue. Thanks!

### 1. Minimal reproduce step (Required)
```
make test
```

```
explain_test.go:294:
Error Trace: explain_test.go:294
explain_test.go:354
Error: Not equal:
expected: "4"
actual : "0"

Diff:
--- Expected
+++ Actual
@@ -1 +1 @@
-4
+0
Test: TestCheckActRowsWithUnistore
Messages: error comparing select * from t_unistore_act_rows
```

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

### 3. What did you see instead (Required)

### 4. What is your TiDB version? (Required)