chore: add root test script and fix LayoutProcessor test

Add `yarn test` that runs tests across all workspaces via
`yarn workspaces foreach`. Fix LayoutProcessor test to call
processMdx with a proper MdxFile instead of the non-existent
process method.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: brunozoric

package.json

@@ -18,6 +18,7 @@
     "format:check": "oxfmt --check",
     "lint": "oxlint",
     "lint:fix": "oxlint --fix",
+    "test": "yarn workspaces foreach -A run test",
     "typecheck": "tsc --noEmit --project scripts/tsconfig.json && tsc --noEmit --skipLibCheck --project generator/tsconfig.json && tsc --noEmit --project docs/tsconfig.json",
     "prepare-release-notes": "node scripts/prepareReleaseNotes.js",
     "get-id": "node -e 'console.log(require(\"@webiny/utils\").mdbid().slice(-8))'",