url, test: fix typo in inspect output, add test

In the string returned from URL.inspect there was an extra semicolon
at the end when showHidden === true. The semicolon has been
removed and a test for the inspect function has been added. The test
parses the returned string, validates all of the contained keys/values
and tests logic related to the showHidden option.

PR-URL: https://github.com/nodejs/node/pull/10231


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Joyee Cheung <joyeec9h3@gmail.com>
Reviewed-By: Evan Lucas <evanlucas@me.com>