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

[lancewillett]

Updated with a non-rewriting merge of current trunk (d39fddb) and moved post-thumbnails and foobar cleanup into unconditional Tests_Theme_Support::tear_down() cleanup. The in-test removal assertions remain unchanged.

Validation completed:

• PHPCS on all changed PHP files
• Focused template and theme tests: 116 tests, 556 assertions
• Randomized themes group with seeds 13194 and 13195: 627 tests, 7,276 assertions each (15 skipped)
• git diff --check

[github-actions]

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[github-actions]

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Props jonsurrell, lancewillett.

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[lancewillett]

Reviewed a922788; no blocking findings. The cleanup restores theme state before the parent teardown, and the negative global-styles assertion now uses an unregistered block fixture.

Validation:

• All three affected classes passed in single-site and multisite: 116 tests and 556 assertions each.
• The themes group passed with random seeds 13194 and 13195: 633 tests and 7,289 assertions each, with 15 skips.
• PHPCS passed all three changed PHP files.

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_{Adversarial review · gpt-6}