Tests: Improve theme test isolation

[sirreal]

Summary

• Uses a genuinely unregistered block fixture for negative global-style assertions and restores theme support, stylesheet, filter, and theme-JSON cache state.
• Keeps the changes confined to PHPUnit tests and fixtures.

Verification

• 100 consecutive fresh randomized themes-group runs passed on the final combined campaign head; each completed 625 tests and 7,297 assertions.
• Exact seeds 1789044001 through 1789044100 all completed within the 30-second hard bound.
• The final gate also covered the non-querying locale restoration carried by the l10n PR; the theme fixture changes had already passed the original 50-run isolated gate.
• 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