test: better assertion for async hook tests

The existing assertion was misleading to whether there were too few or
too many events of a particular type. Improve the assertion message.

PR-URL: https://github.com/nodejs/node/pull/27601


Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>