test: add coverage for vm's breakOnSigint option

The breakOnSigint option follows different code paths, depending
on the number of listeners for SIGINT. This commit updates the
existing test to vary the number of SIGINT handlers.

PR-URL: https://github.com/nodejs/node/pull/12512


Reviewed-By: David Cai <davidcai1993@yahoo.com>
Reviewed-By: James M Snell <jasnell@gmail.com>