redo(ticdc): Fix RedoLogWriter unit test (#11853)

[ti-chi-bot]

This is an automated cherry-pick of #11853

What problem does this PR solve?

Issue Number: close #11852

What is changed and how it works?

A select blocks until one of its cases can run, then it executes that case. It chooses one at random if multiple are ready.
In these cases, encodingWorkerGroup may run WriteEvents or FlushLog successfully even if encoding worker is closed.

select {
case <-ctx.Done():
	return ctx.Err()
case err := <-e.closed:
	return errors.WrapError(errors.ErrRedoWriterStopped, err, "encoding worker is closed")
case e.inputChs[idx] <- event: // executes even if encoding worker is closed
	return nil
}

Check List

Tests

• Unit test

Questions

Will it cause performance regression or break compatibility?

Do you need to update user documentation, design documentation or monitoring documentation?

Release note

`None`.

[ti-chi-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: wk989898

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[wk989898]

/retest

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

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Additional details and impacted files

Components	Coverage Δ
cdc	61.7807% <0.0000%> (?)
dm	51.0772% <0.0000%> (?)
engine	63.4514% <0.0000%> (?)

Flag	Coverage Δ
unit	57.4189% <ø> (?)

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