Fix a11y contrast for pre-test message in flutter_test

[ashoknarayan]

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.

Works done

• The pre-test message "Test starting..." had a contrast ratio of 2.99:1 against the background.
• This PR slightly adjusts the text color to 0xFF8F7FFF to meet the 3:1 minimum contrast ratio for accessibility.
• Tested by running flutter_test.

[google-cla]

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[gemini-code-assist]

Code Review

This pull request correctly fixes an accessibility contrast issue for test messages. However, it also includes unrelated changes to the deprecation message format. To maintain a clean commit history, it's best practice to keep pull requests focused on a single logical change. I've added a comment recommending that the unrelated changes be moved to a separate pull request. If the PR is not split, the description should be updated to mention all the changes.

[gemini-code-assist]

This change, which updates the error message for deprecation notices, and the related documentation update in docs/contributing/Tree-hygiene.md, are unrelated to the main purpose of this pull request (fixing an accessibility contrast issue). To maintain a clean and understandable commit history, please move these changes to a separate pull request.

[fluttergithubbot]

An existing Git SHA, 1c1c83f3d1971c20e544a305c5b42ef0b6ded059, was detected, and no actions were taken.

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