test: improve unexpected warnings error

If someone adds an `expectsWarning` listener without handling all
warning triggered in that test file, it'll result in a cryptic error
message. This improves the situation by providing an explicit error
about the unexpected warning.

PR-URL: https://github.com/nodejs/node/pull/28138


Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Yongsheng Zhang <zyszys98@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>