docs/design: add proposal for restructure tests

[tisonkun]

What problem does this PR solve?

Issue Number: close #26023

This pull requests brings the design document for restructure tests, focusing on unit tests and integration tests, especially on separating integration_test.go as well as unifying test infrastructure.

Release note

• No release note

[tisonkun]

/cc @tiancaiamao @xhebox @disksing

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[tiancaiamao]

/merge

[ti-chi-bot]

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

Commit hash: 1d6b0af

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