test: enable PostCSS multi-config test in Turbopack production mode
Remove the isNextDev restriction so the test also runs during next build
(Turbopack production). The test was previously limited to dev mode out
of caution after a production panic; the test now verifies per-directory
PostCSS config resolution works in both dev and start modes.
Co-Authored-By: Claude <noreply@anthropic.com>