unstable test in the ddl.TestCancel

[hawkingrei]

Bug Report

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1. Minimal reproduce step (Required)

[2022-07-26T09:15:29.490Z]     testkit.go:310: 
[2022-07-26T09:15:29.490Z]         	Error Trace:	testkit.go:310
[2022-07-26T09:15:29.490Z]         	            				cancel_test.go:327
[2022-07-26T09:15:29.490Z]         	Error:      	An error is expected but got nil.
[2022-07-26T09:15:29.490Z]         	Test:       	TestCancel
[2022-07-26T09:15:29.490Z]         	Messages:   	sql: alter table t_partition add primary key c3_index (c1)

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

3. What did you see instead (Required)

4. What is your TiDB version? (Required)

[D3Hunter]

meet again with a different error, the timeline that might trigger this is

• we run prepare sql alter table t add index mul_idx2(c1) and it enter synced state, so the MustExec return, but the ddl worker hasn't return
• we set hook
• ddl worker call OnJobUpdated, but we don't expect the call here, the check fail

    testutil.go:118: 
        	Error Trace:	pkg/ddl/testutil/testutil.go:118
        	Error:      	not multi-schema change(sql: alter table t drop index mul_idx1, drop index mul_idx2, cancel state: [delete reorganization write only])
        	Test:       	TestCancel
        	Messages:   	use model.SchemaState as the cancel state instead