test: add two test cases for querystring

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

PR-URL: https://github.com/nodejs/node/pull/11551


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Yuta Hiroto <hello@about-hiroppy.com>
Reviewed-By: Joyee Cheung <joyeec9h3@gmail.com>