Also fix JSON schema, that was just invalid.

Now we have a schema that renders properly on
https://lab.nexedi.com/bk/rjs_json_form and is similar to the one from
erp5, so maybe one day we can have a convention on erp5 testnode to push
the URL of a selenium server as test-runner.server-url . In any case,
it's good for now to have consistency.

Most of this work comes from Boris Kocherov from https://www.raskon.org/
Thanks !