Tests: Reset comment globals between tests

[sirreal]

Summary

• Clears the global comment fixture after author-URL-link tests.
• Resets $in_comment_loop with the existing comment-loop globals between tests so an empty wp_list_comments() call cannot affect later permalink logic.
• Keeps the changes confined to PHPUnit tests and fixtures.

Verification

• Replayed the original failing seed and minimized it to the comments-template contaminator followed by the comment-link consumer.
• 100 consecutive fresh randomized comment-group runs passed on the combined campaign branch after the final fix; each completed 627 tests and 1,530 assertions. The relevant fixture patch matches this isolated branch.
• Seeds 1789011001 through 1789011100 all exited cleanly.
• PHP syntax checks and git diff --check passed.

Trac: https://core.trac.wordpress.org/ticket/65893

[github-actions]

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