test: refactor test-querystring

- change URIError constructor to regular expression in assert.throws
- use block-scope for tests that spans multiple statements

PR-URL: https://github.com/nodejs/node/pull/12661


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: David Cai <davidcai1993@yahoo.com>