test/commands_spec: write files to tmpdir

[cho-m]

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Writing files to main HOMEBREW_LIBRARY path can cause race conditions like with SimpleCov.

Seen in #21120.

[Copilot]

Pull request overview

This PR refactors test file creation in commands_spec.rb to use a temporary directory instead of writing directly to the main HOMEBREW_LIBRARY path. This change prevents race conditions, particularly with SimpleCov as mentioned in the related issue #21120.

• Introduces tmpdir-based file creation for test isolation
• Uses constant stubbing to redirect Commands module paths to test directories
• Updates test expectations to use stubbed constants instead of hardcoded paths

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[MikeMcQuaid]

Good catch, thanks @cho-m!