test: add missing assertion

This commit adds an assertion to an existing try...catch
statement. Unfortunately, assert.throws() cannot be used
because the operation succeeds on some platforms, throws
EINVAL on some platforms, and throws ENOPROTOOPT on
others.

PR-URL: https://github.com/nodejs/node/pull/15519


Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Yuta Hiroto <hello@about-hiroppy.com>
Reviewed-By: James M Snell <jasnell@gmail.com>