Tests: Isolate block style variation layout test

[sirreal]

Summary

• Runs the layout variation block-gap test in a separate process because the rendering function intentionally retains an otherwise unresettable function-static global-styles snapshot.
• Keeps the changes confined to PHPUnit tests and fixtures.

Verification

• 50 consecutive randomized block-supports-group runs passed; each completed 400 tests and 532 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