tests: Run tagged tests with --changed when shared contexts change

[issyl0]

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• Have you checked to ensure there aren't other open Pull Requests for the same change?
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• Have you successfully run brew lgtm (style, typechecking and tests) with your changes locally?

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• When a shared context file changes, the tests that include that shared context should be run when brew tests --changed is invoked. This ensures that relevant test failures are caught.
• I found this bug while working on sorbet: Bump test/support/helper files to typed: true #22469 where I changed shared contexts, ran brew lgtm which ran brew tests --changed which lulled me into a false sense of security that the changes were fine, until CI ran the full test suite1.
• Add some standard behaviour tests for brew tests --changed as well as there was zero coverage before.

[github-code-quality]

Code Coverage Overview

Languages: Ruby

Ruby / code-coverage/simplecov

The overall coverage remains at 78%, unchanged from the main branch.

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_{Updated May 30, 2026 18:46 UTC
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[Copilot]

Pull request overview

This PR updates brew tests --changed so changes to shared RSpec context files trigger related tagged specs, improving confidence when shared test helpers are modified.

Changes:

• Converts changed-test resolution to support one changed file mapping to multiple specs.
• Adds mappings for integration_test and homebrew_cask shared contexts.
• Adds specs covering direct spec changes, source-to-spec mapping, and shared-context expansion.

Show a summary per file

File	Description
Library/Homebrew/dev-cmd/tests.rb	Expands --changed test selection for shared context files based on tags.
Library/Homebrew/test/dev-cmd/tests_spec.rb	Adds coverage for changed_test_files behavior.

Copilot's findings

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• Files reviewed: 2/2 changed files
• Comments generated: 1

[MikeMcQuaid]

Thanks! One comment, don't need rereview after.