test: stdin is not always a net.Socket

`<`-ing a file into stdin actually results in a `fs.ReadStream`, rather
than a `tty.ReadStream`, and as such does not inherit from net.Socket,
unlike the other possible stdin options.

Refs: https://github.com/nodejs/node/pull/5916
PR-URL: https://github.com/nodejs/node/pull/5935


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>