inspector: wait for both sides closing

Removes race condition when test relied on both sides of the socket
to be closed on the same UV event loop iteration.

Fixes: nodejs/node#8498
PR-URL: https://github.com/nodejs/node/pull/8505


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Saúl Ibarra Corretgé <saghul@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-by: Fedor Indutny <fedor@indutny.com>