test: mount PHPUnit bootstrap failure fixtures

[chubes4]

Summary

• mount a readable plugin fixture at the wordpress.phpunit default cwd and test-root path
• install and mount the existing PHPUnit Playground harness with canonicalizable command arguments
• validate managed install-stage fatal and exit diagnostics while preserving parse-error and secret-redaction coverage

Verification

• npm run build
• npm run test:playground-phpunit-bootstrap-failure-integration
• npm run test:playground-phpunit-readonly-cache-integration
• npm run test:phpunit-structured-evidence
• npm run test:phpunit-runtime-rejection
• npm run test:phpunit-discovery-only
• npm run check (passes the root build and preceding smoke commands, then stops at the independently tracked playground-command-timeout-smoke failure in Timeout smoke can expire before fake Playground command starts #1904)
• npm run test:phpunit-runtime-failure-diagnostics remains independently failing as tracked in PHPUnit runtime diagnostics test expects wrapper bypassed by runtime-only bootstrap #2138

Fixes #2139