Tests: Reset permalinks before database rollback

[sirreal]

Summary

• Resets permalink structure at the start of core test teardown so in-memory rewrite state remains synchronized with the database state restored by rollback.
• Keeps the changes confined to PHPUnit tests and fixtures.

Verification

• 50 consecutive randomized post-group runs passed; each completed 933 tests and 2,641 assertions with one expected skip.
• 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