[Impeller] attempt to get validation errors from CI unittests.

[jonahwilliams]

Enables Vulkan validation layers for macOS and linux builds, enables macOS validation for macOS builds.

Fixes https://github.com/flutter/flutter/issues/144967

There may still be remaining issues with macOS validation, I need to verify I can reproduce https://github.com/flutter/flutter/issues/143324 . I plan to do that separately

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Changes reported for pull request #51341 at sha c53ff77e3eb062a9d688eff1ad3cef444ef2db19