Encourage splitting large test files in testing documentation 2

[crackedhandle]

This PR updates the testing documentation to encourage contributors to split large test files into smaller, feature-focused files.
its a PR2

Smaller test files improve readability, navigation, maintainability, and reviewability.

Fixes #181819

[fluttergithubbot]

An existing Git SHA, dddce280e1861b6838637f82634b6170ac13b988, was detected, and no actions were taken.

To re-trigger presubmits after closing or re-opeing a PR, or pushing a HEAD commit (i.e. with --force) that already was pushed before, push a blank commit (git commit --allow-empty -m "Trigger Build") or rebase to continue.

[gemini-code-assist]

Code Review

This pull request updates the testing documentation to encourage splitting large test files, which is a valuable improvement for code maintainability. The added section is clear and provides helpful examples. I've provided one suggestion to enhance the wording for conciseness and to include a direct link to the corresponding section in the repository's style guide, reinforcing the documented best practice.

[fluttergithubbot]

An existing Git SHA, c3b9c96c7527084f9079071fac1da1c08a66ab12, was detected, and no actions were taken.

To re-trigger presubmits after closing or re-opeing a PR, or pushing a HEAD commit (i.e. with --force) that already was pushed before, push a blank commit (git commit --allow-empty -m "Trigger Build") or rebase to continue.

[fluttergithubbot]

An existing Git SHA, f6c49aac57196ed00bb5799ea78f8e7f202cbbba, was detected, and no actions were taken.

To re-trigger presubmits after closing or re-opeing a PR, or pushing a HEAD commit (i.e. with --force) that already was pushed before, push a blank commit (git commit --allow-empty -m "Trigger Build") or rebase to continue.

[crackedhandle]

@Piinks @loic-sharma

[Piinks]

LGTM thanks for opening a new PR!

[chunhtai]

LGTM