test(e2e): fix title suffix case occasionally failed

[chenjiahan]

Summary

Fix title suffix case occasionally failed, check the generated HTML files instead of go to the pages.

https://github.com/web-infra-dev/rspress/actions/runs/8832509957/job/24249962993

Checklist

• [x] Tests updated (or not required).
• [ ] Documentation updated (or not required).

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