Add w/o grouping run in golden test

[lizard-boy]

Original PR: uber-go/nilaway#253

[greptile-apps]

PR Summary

Overall, the PR introduces a matrix strategy in the golden test workflow to support both grouped and ungrouped error outputs, updating both tests and main functionality accordingly.

• Modified file: /.github/workflows/golden-test.yml to add a matrix with entries for grouping and non-grouping runs.
• Updated file: tools/cmd/golden-test/main_test.go to hardcode and validate the grouping flag behavior.
• Altered file: tools/cmd/golden-test/main.go to accept and pass the grouping flag dynamically.
• Potential issue: Standard log functions are used for error logging instead of the custom logger.error module.

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