test: cover vm.runInNewContext()

There is currently one if branch missing coverage in lib/vm.js.
This commit adds a test to cover the case where the third
argument to runInNewContext() is a string.

PR-URL: https://github.com/nodejs/node/pull/16906


Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Yuta Hiroto <hello@about-hiroppy.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Franziska Hinkelmann <franziska.hinkelmann@gmail.com>