test: fix scriptParsed event expectations

As per Node.js docs, vm.Script instance is not bound to any context.

However, this test was expecting otherwise and depended on
implementation details which are going to change.

Refs: https://chromium-review.googlesource.com/c/v8/v8/+/1013581

PR-URL: https://github.com/nodejs/node/pull/21079


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Yang Guo <yangguo@chromium.org>