ddl: make several test cases stable for make ut

[tiancaiamao]

What problem does this PR solve?

Issue Number: close #31085
A subtask for #30822

Problem Summary:

cd ddl;
go test -run TestSchema

Similar to issue #31085, the error log is

    util_test.go:173:
                Error Trace:    util_test.go:173
                                                        schema_test.go:173
                Error:          Expected nil, but got: [ddl:-1]failed to notify PD the placement rules: infoSyncer is not initialized
                Test:           TestSchema

What is changed and how it works?

Check List

The test case can't run indenpendently, because the infoSyncer global variable is not set.

Call testkit.CreateMockStore(t) to make it able to run

go test -run TestSchema

Tests

• Unit test
• Integration test
• Manual test (add detailed scripts or steps below)
• No code

Side effects

• Performance regression: Consumes more CPU
• Performance regression: Consumes more Memory
• Breaking backward compatibility

Documentation

• Affects user behaviors
• Contains syntax changes
• Contains variable changes
• Contains experimental features
• Changes MySQL compatibility

Release note

None

[ti-chi-bot]

[REVIEW NOTIFICATION]

This pull request has been approved by:

• hawkingrei
• xhebox

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[sre-bot]

Code Coverage Details: https://codecov.io/github/pingcap/tidb/commit/f45f489d550e9f228737ca9adfd46ce83f06700b

[xhebox]

/merge

[ti-chi-bot]

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

Commit hash: 6f673cf

[hawkingrei]

/run-check_dev_2

[purelind]

/run-unit-test

[purelind]

/run-unit-test

[hawkingrei]

/run-mysql-test

[ti-chi-bot]

@tiancaiamao: Your PR was out of date, I have automatically updated it for you.

At the same time I will also trigger all tests for you:

/run-all-tests

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