[INTERNAL] Disable AVA worker threads

This also solves the Node 20 issue initially addressed via #511.
However, it also addresses another issue that mainly appears in the
ui5-builder projects, see SAP/ui5-builder#921.

Tests may run a bit slower, but more stable.

JIRA: CPOUI5FOUNDATION-612

package.json

@@ -73,7 +73,8 @@
  "nodeArguments": [
  "--loader=esmock",
  "--no-warnings"
- ]
+ ],
+ "workerThreads": false
  },
  "nyc": {
  "reporter": [