test: add two test cases for querystring

Test cases:

• Cover untested branch in the state machine with the strings that its first letter is + in the key/value.
• qs.unescapeBuffer shouldn't decode + to space.

This test increases the coverage of querystring.js:

• https://coverage.nodejs.org/coverage-d42296bd985ae7df/root/querystring.js.html

The following branches will be skipped:

• https://github.com/nodejs/node/blob/d4e1eaf43cc01ba1d151697f916abff4524062eb/lib/querystring.js#L64-L65
• https://github.com/nodejs/node/blob/d4e1eaf43cc01ba1d151697f916abff4524062eb/lib/querystring.js#L405-L406
• https://github.com/nodejs/node/blob/d4e1eaf43cc01ba1d151697f916abff4524062eb/lib/querystring.js#L413-L414

Checklist

• make -j4 test
• tests are included
• commit message follows commit guidelines

Affected core subsystem(s)

test