test: ensure failed assertions cause build to fail

This updates the test in `test/parallel/test-assert-async.js` to add an
assertion that the Promises used in the test end up fulfilled.
Previously, if an assertion failure occurred, the Promises would have
rejected and a warning would have been logged, but the test would still
have exit code 0.

PR-URL: https://github.com/nodejs/node/pull/19650


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>