Tests: Improve isolation for randomized query tests

[sirreal]

Summary

• Restores sticky and canonical-query fixtures and uses the recognized case-sensitive ID orderby value to make child-post selection deterministic.
• Keeps the changes confined to PHPUnit tests and fixtures.

Verification

• 50 consecutive randomized query-group runs passed; each completed 1,892 tests and 4,420 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