test: add and assert readable/writable arguments

Currently the readable and writable arguments are not specified in the
req.oncomplete method. Adding and asserting that they are always true
(which is always the case for TCP). This might seem unnecessary but it
can't hurt to have them to pickup any breaking modifications made to
ConnectionWrap::AfterConnect in the future.

PR-URL: https://github.com/nodejs/node/pull/8815


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: James M Snell <jasnell@gmail.com>