test: improve async hooks test error messages

Improve error messages in the async hooks tests, mostly by removing
unhelpful `message` parameters for assertions.

PR-URL: https://github.com/nodejs/node/pull/13243


Reviewed-By: Kunal Pathak <kunal.pathak@microsoft.com>
Reviewed-By: Andreas Madsen <amwebdk@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Trevor Norris <trev.norris@gmail.com>