test: add coverage for cli.ts

[JoshuaKGoldberg]

PR Checklist

• Addresses an existing open issue: fixes 🧪 Testing: Fill in test coverage for cli.ts and utils.ts #177
• That issue was marked as status: accepting prs
• Steps in CONTRIBUTING.md were taken

Overview

Adds two unit test files. Refactors execOrThrow to create exec lazily, to make it easier to mock.

[JoshuaKGoldberg]

@all-contributors please add @JoshuaKGoldberg for test.

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[allcontributors]

@JoshuaKGoldberg

I've put up a pull request to add @JoshuaKGoldberg! 🎉

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