Refactor collapse issue comments end-to-end test

What does this MR do?

This refactoring:

• Better separates scopes (pre-conditions on before, tests on it)
• Removes an unnecessary expectation
• Replaces other unnecessary expectations by page.assert_text
• Moves variables closer to where they're used

Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry for user-facing changes, or community contribution. Check the link for other scenarios.
• [-] Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• [-] Database guides
• [-] Separation of EE specific content