test: improve zlib-from-gzip-with-trailing-garbage

* use assert.strictEqual instead of assert.equal
* add RegExp in second argument of assert.throws
* validate error message and code

PR-URL: https://github.com/nodejs/node/pull/10674


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>