test: remove literals that obscure assert messages

Remove string literals as messages to `assert.strictEqual()`. They can
be misleading here (where perhaps the reason an assertino failed isn't
that the deleter wasn't called but rather was called too many times.

PR-URL: https://github.com/nodejs/node/pull/17642


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Jon Moss <me@jonathanmoss.me>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Anatoli Papirovski <apapirovski@mac.com>
Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>