Tests: Restore themes after block tests

[sirreal]

Summary

• Restores the incoming stylesheet after block-template and editor fixtures switch themes.
• Applies the same teardown discipline to the remaining blocks-group fixtures that switch themes.
• Prevents stale Theme JSON state from changing later block rendering.
• Keeps the changes confined to PHPUnit tests and fixtures.

Verification

• Minimized two independent contaminators of the block-rendering columns case and replayed both original failing seeds.
• 100 consecutive fresh randomized blocks-group runs passed on the combined campaign branch after the complete theme-switch audit; each completed 1,080 tests and 2,825 assertions. Both relevant patch IDs and all seven changed file trees match this isolated branch.
• Seeds 1789029001 through 1789029100 all exited cleanly.
• PHP syntax checks and git diff --check passed.

Trac: https://core.trac.wordpress.org/ticket/65893

[github-actions]

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