test: add regex check in test-vm-is-context

Use a regex to validate the error message.

PR-URL: https://github.com/nodejs/node/pull/12785


Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: James M Snell <jasnell@gmail.com>