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

• 50 consecutive randomized themes-group runs passed; each completed 625 tests and 7,297 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