chore: Backport #19 from main to the v2 branch

[smockle]

Part of https://github.com/github/continuous-ai-for-accessibility/issues/76 (Hubber access only)

Follow-up to #19

[Copilot]

Pull Request Overview

This PR backports cleanup improvements from the main branch to the v2 branch, specifically enhancing the test workflow's ability to manage GitHub resources during test cleanup.

• Centralizes the testing repository configuration using an environment variable
• Adds automatic branch deletion after closing pull requests during cleanup
• Improves resource management by ensuring test branches are properly removed

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[Copilot]

The branch deletion logic could potentially delete protected or important branches if the PR URL parsing fails or returns unexpected branch names. Consider adding validation to ensure only test-related branches are deleted, such as checking for a specific prefix pattern.

Suggested change

	echo "Deleting branch: $branch"
	gh api -X DELETE "repos/${{ env.TESTING_REPOSITORY }}/git/refs/heads/$branch" || echo "Failed to delete branch: $branch"
	if [[ "$branch" == test-* || "$branch" == ci-* ]]; then
	echo "Deleting branch: $branch"
	gh api -X DELETE "repos/${{ env.TESTING_REPOSITORY }}/git/refs/heads/$branch" || echo "Failed to delete branch: $branch"
	else
	echo "Skipping deletion of non-test branch: $branch"
	fi