[pf-evals] Document improvements and convert timeout to constant

[ninghu]

Description

Please add an informative description that covers that changes made by the pull request and link all relevant issues.

All Promptflow Contribution checklist:

• The pull request does not introduce [breaking changes].
• CHANGELOG is updated for new features, bug fixes or other significant changes.
• I have read the contribution guidelines.
• I confirm that all new dependencies are compatible with the MIT license.
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General Guidelines and Best Practices

• Title of the pull request is clear and informative.
• There are a small number of commits, each of which have an informative message. This means that previously merged commits do not appear in the history of the PR. For more information on cleaning up the commits in your PR, see this page.

Testing Guidelines

• Pull request includes test coverage for the included changes.

[github-actions]

promptflow-evals test result

 12 files   12 suites   1h 22m 31s ⏱️
 49 tests  48 ✅  1 💤 0 ❌
588 runs  576 ✅ 12 💤 0 ❌

Results for commit 56d0740.

♻️ This comment has been updated with latest results.