test: fix unstable CI caused by concurrent unit test (#1541)

[ti-srebot]

cherry-pick #1541 to release-2.0
You can switch your code base to this Pull Request by using git-extras:

# In dm repo:
git pr https://github.com/pingcap/dm/pull/1545

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git push git@github.com:ti-srebot/dm.git pr/1545:release-2.0-7c7f66069b96

---

What problem does this PR solve?

in #1539 https://internal.pingcap.net/idc-jenkins/blue/organizations/jenkins/dm_ghpr_test/detail/dm_ghpr_test/8606/pipeline , one unit test is disturbed by other

[2021-03-29T03:59:27.882Z] FAIL: optimist_test.go:148: testOptimist.TestOptimist
[2021-03-29T03:59:27.882Z] 
[2021-03-29T03:59:27.882Z] optimist_test.go:149:
[2021-03-29T03:59:27.882Z]     t.testOptimist(c, noRestart)
[2021-03-29T03:59:27.882Z] optimist_test.go:351:
[2021-03-29T03:59:27.882Z]     c.Assert(opm, HasLen, 0)
[2021-03-29T03:59:27.882Z] ... obtained map[string]map[string]map[string]map[string]optimism.Operation = map[string]map[string]map[string]map[string]optimism.Operation{"task":map[string]map[string]map[string]optimism.Operation{"mysql-replica-1":map[string]map[string]optimism.Operation{"foo":map[string]optimism.Operation{"bar-1":optimism.Operation{ID:"task-`db`.`tbl`", Task:"task", Source:"mysql-replica-1", UpSchema:"foo", UpTable:"bar-1", DDLs:[]string{"ALTER TABLE bar ADD COLUMN c1 INT"}, ConflictStage:"none", Done:false}}}}}
[2021-03-29T03:59:27.882Z] ... n int = 0

What is changed and how it works?

seems we should use SerialSuites

Check List

Tests

• No code

Code changes

Side effects

Related changes

• Need to cherry-pick to the release branch

[lance6716]

/lgtm

[GMHDBJD]

/lgtm

[ti-chi-bot]

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• GMHDBJD
• lance6716

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[lance6716]

/merge

[ti-chi-bot]

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

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Commit hash: 18aab7f