test: remove custom message from assertion

The custom message means that the values involved are not reported.
Remove the custom message for a more detailed error message.

PR-URL: https://github.com/nodejs/node/pull/16824


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Anatoli Papirovski <apapirovski@mac.com>
Reviewed-By: Gireesh Punathil <gpunathi@in.ibm.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>