ddl: make TestNotifyDDLJob stable

[zimulala]

What problem does this PR solve?

Issue Number: close #xxx

Problem Summary:

[2021-03-09T09:00:56.914Z] ----------------------------------------------------------------------
[2021-03-09T09:00:56.914Z] FAIL: ddl_worker_test.go:80: testDDLSuite.TestNotifyDDLJob
[2021-03-09T09:00:56.914Z]
[2021-03-09T09:00:56.914Z] ddl_worker_test.go:128:
[2021-03-09T09:00:56.914Z]     c.Fatal("do not get the add index job notification")
[2021-03-09T09:00:56.914Z] ... Error: do not get the add index job notification
[2021-03-09T09:00:56.914Z]

What is changed and how it works?

Proposal: xxx

How it Works:

Ensure that the notification is not handled by the worker's "start".

Check List

Tests

• No code

Release note

• No release note.

[zimulala]

/cc @xhebox @bb7133 @crazycs520

[AilinKid]

/lgtm

[xhebox]

/lgtm

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[REVIEW NOTIFICATION]

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• AilinKid
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[ngaut]

/merge

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In response to this:

/merge

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[xhebox]

/merge

[ti-chi-bot]

This pull request has been accepted and is ready to merge.

Commit hash: b8c5f03