Tests: Reset global comment after author URL tests

[sirreal]

Summary

• Clears the global comment fixture during teardown so the no-comment case cannot inherit a comment from a preceding test.
• Keeps the changes confined to PHPUnit tests and fixtures.

Verification

• 50 consecutive randomized comment-group runs passed; each completed 627 tests and 1,530 assertions.
• Replayed the original failing seed and focused contaminator/consumer cases during diagnosis.
• PHP syntax checks passed for every changed PHP file.
• git diff --check passed.

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