test: complete coverage of querystring

PR-URL: https://github.com/nodejs/node/pull/38520
Refs: https://coverage.nodejs.org/coverage-68e6673224365120/lib/querystring.js.html#L179


Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Zijian Liu <lxxyxzj@gmail.com>
Reviewed-By: Darshan Sen <raisinten@gmail.com>