assert: fix .throws operator

assert.throws and assert.doesNotThrow set the operator to a
internal function. That was by accident and originally the operator
was undefined. This changes it to show "throws" or "doesNotThrow".

PR-URL: https://github.com/nodejs/node/pull/17575


Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Anatoli Papirovski <apapirovski@mac.com>