server/tests/cursor: fix cursor fetch error flacky test (#50969)

[ti-chi-bot]

This is an automated cherry-pick of #50969

What problem does this PR solve?

Issue Number: close #47029

Problem Summary:

I finally found the root cause of this flacky test. It's easy to understand:

Success Path:

1. Execute the statement
2. Enable the failpoint
3. Fetch the statement
   3.1. The rowContainerReader worker works in the background, and return the error specified by failpoint

Fail Path:

1. Execute the statement
2. The rowContainerReader worker works in the background. At this time, the failpoint is still not enabled.
3. Enable the failpoint
4. Fetch the result, but get no error.

The rowContainerReader worker can run right after Execute command, because it has a channel buffer whose size is twice as the chunk size. By default, it's just 2048 (equals the whole table size in this test 🤦). Therefore, another fix of this issue is to increase the table size from 2048 to a higher value. But I choose to enable failpoint before executing the statement.

What changed and how does it work?

Enable the failpoint before executing the statement, so that it'll definitely trigger the failpoint.

Check List

Tests

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

  • I checked and no code files have been changed.

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

Please refer to Release Notes Language Style Guide to write a quality release note.

None

[ti-chi-bot]

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[ti-chi-bot]

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@ti-chi-bot: The following tests failed, say /retest to rerun all failed tests or /retest-required to rerun all mandatory failed tests:

Test name	Commit	Details	Required	Rerun command
idc-jenkins-ci-tidb/build	cc61c88	link	true	/test build
idc-jenkins-ci-tidb/unit-test	cc61c88	link	true	/test unit-test
idc-jenkins-ci-tidb/check_dev	cc61c88	link	true	/test check-dev
idc-jenkins-ci-tidb/check_dev_2	cc61c88	link	true	/test check-dev2

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