Playwright: Fix assertUnreadLessThan and assertUnreadGreaterThan

[florianduros]

Checklist

• Tests written for new code (and old code if feasible)
• Linter and other CI checks pass
• Sign-off given on the changes (see CONTRIBUTING.md)

toBeGreaterThan and toLesserThan are non retry assertion, so we should use expect.poll to add a retry mechanism.

Part of element-hq/element-web#25449

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This change is marked as an internal change (Task), so will not be included in the changelog.