test: fix flaky test-vm-sigint-existing-handler

Set the `SIGUSR2` handler before spawning the child process to make sure
the signal is always handled.

Ref: https://github.com/nodejs/node/pull/7854
Fixes: https://github.com/nodejs/node/issues/7981
PR-URL: https://github.com/nodejs/node/pull/7982


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>