test: add cases for unescape & unescapeBuffer

Increase coverage of querystring:

• https://coverage.nodejs.org/coverage-4cafa60c99f0d755/root/querystring.js.html

This test case will cover these lines:

• https://github.com/nodejs/node/blob/4cafa60c99f0d755b1df3a29357e19a92b97eed8/lib/querystring.js#L114
• https://github.com/nodejs/node/blob/4cafa60c99f0d755b1df3a29357e19a92b97eed8/lib/querystring.js#L462

Checklist

• make -j4 test (UNIX), or vcbuild test (Windows) passes
• tests and/or benchmarks are included
• commit message follows commit guidelines

Affected core subsystem(s)

test, querystring