flaky test TestIndexChange

[hawkingrei]

Bug Report

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1. Minimal reproduce step (Required)

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

3. What did you see instead (Required)

     index_change_test.go:74: 
        	Error Trace:	pkg/ddl/index_change_test.go:74
        	            				pkg/ddl/util/callback/callback.go:124
        	            				pkg/ddl/job_table.go:448
        	            				pkg/util/wait_group_wrapper.go:154
        	            				src/runtime/asm_amd64.s:1650
        	Error:      	Not equal: 
        	            	expected: 3
        	            	actual  : 0
        	Test:       	TestIndexChange
        	Messages:   	job's row count 0 != 3 

4. What is your TiDB version? (Required)