test: rewrite assert message

`test/parallel/test-zlib-unzip-one-byte-chunks.js` uses a literal
string to describe the error if the string does not match itself
after zipping and unzipping. Changed to a more descriptive template
literal.

PR-URL: https://github.com/nodejs/node/pull/15879


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Tobias Nießen <tniessen@tnie.de>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>