unstable test in the TestCheckActRowsWithUnistore

[hawkingrei]

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)

[jackysp]

It happened again in #50365 (release-7.1).