Add tests for useTabs hook

[joshfarrant]

Summary

Backfills tests for useTabs hook to bring coverage up to (nearly) 100%.

These tests highlighted some incorrect types in the useTabs hook which I plan to resolve in a subsequent PR (#1138), keeping this PR focused on coverage.

The recent modifications made to this hook in the recently merged Tabs component PR have introduced a line which is proving tricky to test. The subsequent PR mentioned above will address that and bring coverage up to 100%, which isn't possible here as it will require a modification to the hook's implementation.

Supporting resources (related issues, external links, etc)

• Towards https://github.com/github/primer/issues/5412

Contributor checklist

• All new and existing CI checks pass
• Tests prove that the feature works and covers both happy and unhappy paths
• Any drop in coverage, breaking changes or regressions have been documented above
• UI Changes contain new visual snapshots (generated by adding update snapshots label to the PR)
• All developer debugging and non-functional logging has been removed
• Related issues have been referenced in the PR description

Reviewer checklist

• Check that pull request and proposed changes adhere to our contribution guidelines and code of conduct
• Check that tests prove the feature works and covers both happy and unhappy paths
• Check that there aren't other open Pull Requests for the same update/change

[changeset-bot]

⚠️ No Changeset found

Latest commit: a3cd51a

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[github-actions]

🟢 No design token changes found

[github-actions]

🟢 Unit test coverage changes found

Unit test coverage has been updated through this PR.

Changes: 1 new tests, 0 removed tests, 0 improved, 0 decreased

Component/Hook	Statements	Functions	Branches	Change
useTabs	97.9%	100.0%	97.9%	NEW

[github-actions]

🟢 No visual differences found

Our visual comparison tests did not find any differences in the UI.

[Copilot]

Pull Request Overview

This PR adds comprehensive test coverage for the useTabs hook to achieve 100% code coverage. The tests validate all aspects of the hook's functionality including tab activation, keyboard navigation, accessibility attributes, and event handling.

Key Changes

• Creates a complete test suite with 1,229 lines covering all hook behaviors
• Implements a custom hook testing utility that simulates real DOM interactions
• Tests all keyboard navigation patterns, accessibility compliance, and edge cases