Remove divider from widgets tests

[navaronbracke]

This PR removes Divider usages from test/widgets/scroll_view_test.dart and test/widgets/list_view_test.dart.
Usages of Divider() still exist under examples/api and in code samples under lib/widgets, though.

Part of #177415
Part of #180501

If you had to change anything in the flutter/tests repo, include a link to the migration guide as per the breaking change policy.

Pre-launch Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• I read the Tree Hygiene wiki page, which explains my responsibilities.
• I read and followed the Flutter Style Guide, including Features we expect every widget to implement.
• I signed the CLA.
• I listed at least one issue that this PR fixes in the description above.
• I updated/added relevant documentation (doc comments with ///).
• I added new tests to check the change I am making, or this PR is test-exempt.
• I followed the breaking change policy and added Data Driven Fixes where supported.
• All existing and new tests are passing.

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[gemini-code-assist]

Code Review

This pull request effectively removes usages of Divider from several widget tests to reduce dependencies on the material package. The changes correctly replace Divider with SizedBox or custom widget implementations as appropriate. A new file, widget_test_utils.dart, is introduced to provide basic widgets for tests, which is a good step towards better test modularity. The changes are well-executed and align with the PR's goal. I have one suggestion for the new test utility file to improve clarity and maintainability.

[gemini-code-assist]

The function name getRandomTestColor and its documentation are misleading. The function returns a deterministic color from a predefined list based on the index, not a random color. The documentation also incorrectly states that RGB channels are randomized.

To improve clarity and accuracy, I suggest renaming the function to getTestColor and updating the documentation to reflect its actual behavior. This aligns with the style guide's emphasis on readability and useful documentation.

/// Get a deterministic color for use in a widget test.
///
/// The returned color will be fully opaque and is selected from a predefined
/// list of colors based on the given [index].
Color getTestColor(int index) {

References

1. The style guide emphasizes optimizing for readability. The function name getRandomTestColor is misleading as it's not random, which harms readability. ^(link)
2. The style guide requires documentation to be useful. The current documentation is inaccurate and therefore not useful. ^(link)

[justinmc]

LGTM 👍

[justinmc]

Good catch.

[navaronbracke]

Since this has 2 approvals now, can this get an autosubmit as well?