ddl: make ddl test more stable

[crazycs520]

What problem does this PR solve?

[2019-09-29T12:21:59.328Z] ----------------------------------------------------------------------
[2019-09-29T12:21:59.328Z] FAIL: db_test.go:606: testDBSuite2.TestCancelDropTableAndSchema
[2019-09-29T12:21:59.328Z] 
[2019-09-29T12:21:59.328Z] db_test.go:681:
[2019-09-29T12:21:59.328Z]     c.Assert(err.Error(), Equals, "[ddl:12]cancelled DDL job")
[2019-09-29T12:21:59.328Z] ... obtained string = "[domain:2]Information schema is changed. [try again later]"
[2019-09-29T12:21:59.328Z] ... expected string = "[ddl:12]cancelled DDL job"

The cause problem is:

[2019-09-29T12:21:55.954Z] [2019/09/29 20:21:55.827 +08:00] [INFO] [session.go:1859] ["CRUCIAL OPERATION"] [conn=117] [schemaVersion=55] [cur_db=test_drop_db] [sql="create table if not exists t(c1 int, c2 int)"] [user=]
...
...
...
[2019-09-29T12:21:58.812Z] �[0m[2019/09/29 20:21:58.663 +08:00] [INFO] [session.go:1859] ["CRUCIAL OPERATION"] [conn=117] [schemaVersion=55] [cur_db=test_drop_db] [sql="drop table t;"] [user=]

As you can see, the schemaVersion is 55, the session doesn't load the latest schema.

The simplest way is set the ddl lease for another test case too, but this method can only reduce the probability of occurrence.

The best method is force make the session to reload schema, we can add OnChanged method to do domain.reload for TestDDLCallback to fix this problem, but this method is a little complicated.

Let us just change the lease first.

What is changed and how it works?

Check List

Tests

• No code

[sre-bot]

Thanks for your PR.
This PR will be closed by bot because you already had 3 opened PRs, close or merge them before submitting a new one.
#12179 , #12213 , #12484

[sre-bot]

Thanks for your PR.
This PR will be closed by bot because you already had 3 opened PRs, close or merge them before submitting a new one.
#12179 , #12213 , #12484

[zimulala]

LGTM

[tangenta]

LGTM

[codecov]

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[crazycs520]

/run-all-tests

[sre-bot]

Your auto merge job has been accepted, waiting for 12537

[sre-bot]

/run-all-tests

[sre-bot]

@crazycs520 merge failed.